Cost of a Model from Shapeways 
The answer is .. it depends. I say that and people will stop .. blink .. and say .. “Oh. Ok .. I mean .. what would it cost for that lefthanded Widget you made?” … to which I reply .. it still depends. 
Materials 
First you need to determine what material the model will be printed in. I have used two materials from Shapeways – their WSF (White Strong and Flexible) and their FUD (Frosted Ultra Detail). I have mostly used FUD. This is their highest resolution material and most suitable for a model. My opinion is that it works well for O scale – occasionally some slight sanding to remove the small ridges left by the support material during printing. Smaller scales are more ‘iffy’. WSF is a nylon and pretty strong but it has a texture that can cause problems .. it also has less of a resolution than FUD. If used right though it is useful. I have had bridge shoes which represent cast iron that look great (IMO) printed in WSF.
Here is a link to the Shapeways materials that I think would be useful in modeling. I say again, I have only had models printed in WSF and FUD. With that said – I am going to talk about “What it will cost in FUD” – the Shapeways website’s Materials Page goes into GREAT detail on the different materials they have available. 
The determinants … 
When we start trying to determine (determine .. determinants .. get it? heh) what a model will cost us there are several things that need to be taken into consideration:

Real World Examples 
I am in the midst of building a 1:48 scale tramp steamer for my layout and I realized I need Cowl Ventilators. Prior to modern ships with forced ventilation these were ubiquitous. I looked around a bit on the web and found a few for sale here and there – and wasn’t impressed. The ones that were reasonable in price .. and possibly of use to me .. I couldn’t get a large enough photo to see the quality. It seemed to me that I could ‘roll my own’ .. which I did. You can see the process in my blog .. Cowl Ventilators.
I created two different size ventilators for my ship. In the midst of this I was talking to Peter August about these and he mentioned he could use some in N scale. That interested me and I created a N scale version. I uploaded some of this size with the O scale versions .. the extra cost is negligible really .. they may work for N scale .. and they may not. Shrug. Only one way to find out. 
N Scale Cowl Ventilator 
This single Cowl Ventilator is (in) 0.176w x 0.25d x 0.435h (cm) 0.446w x 0.634d x 1.106h. The Volume 0.0400 cm^{3}. When uploaded it errored out in the initial checks with “Too small” or in the Wall Thickness Check with “Thin Walls” or complete failure. The only one it passed was for FUD. This makes sense since this was the material I designed it for.
Side note: Looking at the photo the question will come up why are the walls so thick? That is 0.015 in. FUD has a minimum wall thickness of .3 mm / 0.0118 in. I tried the model with 0.012 in. walls and got a Thin Wall warning. I reworked it with 0.015 in. walls and had no problem. I have 0.020 in. walls for the larger Cowl Ventilators I made in O scale. That will be less noticeable but I think that some slight sanding will fix even that. 
Notice that the volume for this single ventilator is 0.04 ^{3}. FUD costs $3.49 ^{3}. Multiply 0.04 x $3.49 and you get $.1396. Shapeways rounds up so it will cost you 14¢ each .. for the material. The handling for this one item would be $5. The shipping let’s say is the same as the order I placed for multiple items, $4.99 – this means that ordering a single N Scale Cowl Ventilator would cost you $.14 + $5.00 + $4.99 .. or .. $10.13
Say You wanted six of these .. and for SOME reason you placed an order for SIX items. That would cost you (6 x $.14 = $.84) + (6 x $5 = $30) + $4.99 = $35.83. Divide that back by six and you get $35.83 / 6 = $5.97 each!! Wowziers. Ok. That’s silly .. few people would go that route. The answer then is .. put multiple items in one mesh. If you put six cowl ventilators into one mesh the numbers come out a bit different. (6 x $.14 = $.84) + $5 + $4.99 = $10.83 .. divide that by six and you get .. $10.83 / 6 = $1.80 .. still not cheap but not crazy expensive. What if you maxed out the bounding box with N scale cowl ventilators? With a Max Bounding Box of 127 x 178 x 152 mm we have a volume of 3436 cm^{3}. Our ventilator occupies a volume (wxdxh) of 0.32 cm^{3}. That means theoretically we could print 10,737 ventilators. That would total 429.48 cm^{3} .. and at $3.49 cm^{3} would be about $1500 !!! .. and the cost per would drop to about the cost of the material per or 14¢ each .. So the idea is to find a happy medium between too much and WAY too much! Examples (assuming the shipping remains at $4.99) – Number – Total Cost / Cost each
The point being that “What it costs?” .. depends. In all these cases it *assumed* that I have uploaded a mesh with 10, 50, 100 or 1000 ventilators in one mesh. The prices shown are the cost to me. Selling these I would add a 10% markup. So for example the 100ea would cost me $23.99 or $0.24 each. I would add my 10% markup of $2.40 so the total to you would be $23.99 + $2.40 = $26.39 or $0.27 each. 
Side note. I added twelve each of the N scale ventilators to my order. Amortizing for six ea. (Which Peter August needs) – material cost, handing and shipping – those six .. would be $.84 cost for material .. $1.19 including the proportion of handling and shipping. I would add a 10% markup – my profit of $.12 for a total cost of $1.31 .. plus shipping/handling (meaning cost of well .. the shipping package and cost of stamps). 
Note that for other materials via the link there are costs for handling and cost per cubic cm. I have .. IMO .. have great results in O scale using the WSF to replicate cast iron.