Can I make my custom weapon cost 2 or 3 credits instead of 1?
No. I don't want the equipment menu to turn into a CS:S-like store. The decision a player with a credit makes right now is "which equipment is going to help me most". If equipment starts having variable prices, the decision suddenly involves options such as saving for more expensive equipment, and which equipment is the most price-efficient. This seemingly small change will add significant complexity.

In short: this is intentional and will not change.

Balance your weapon for 1 credit. If you're looking for an easy option, limit the weapon's ammo and let players buy another one to get more shots once they run out.