It's perfectly feasible to put several pieces into a single file as long as the total polygon count doesn't go over 500,000. For a few axes, I daresay you'll be safe.
There's no way of knowing what it'll cost, though, until you know the volume of the overall model. In the case of the stainless steel, it costs $10 per cubic centimetre of material used (prices for the various materials, as well as printing guidelines, can be found
here).
Once you've made your model and upload it to the site, it'll tell you how much it'll cost to order. If it's too much for you, you can always modify it (remove an axe or two, or scale them down, for instance), and re-upload it; there's no obligation to buy straight away just because you've uploaded it.
Andy