Take advantage of technology
If cost is a concern, our best advice is to focus your budget on the main announcement/invitation. All other details can be put on a wedding website, and replies can be handled via phone, e-mail, or online. Some good resources for wedding websites and the handling of online replies:
Instead of envelope printing, consider a custom stamp
Another way to save both time and money (win!) is to order a matching address stamp to go with your suite instead of having your envelopes printed. You'll still have to hand-write the guest addresses, but you'll save a lot of time on the return addresses (and these can be used for your reply card and thank you note envelopes, too).
Order a few extras at the beginning
If there's any chance you're going to need more invitations, order a few extras at the start. If you end up needing to print additional invites, you'll pay additional shipping costs (and if you're last-minute about it, you could also end up paying rush fees).
Avoid square invitations
It costs more to mail square envelopes than rectangular ones. (They have to be hand-canceled and therefore come with a postage surcharge.) If you're really set on a square invitation you might consider requesting rectangular envelopes if budget is a major concern.
Include reception info on your invitation
It used to be considered improper to include your reception information on the main ceremony invitation. That's where reception cards came from. But today, it's far more common to include both ceremony and reception info on the main invitation, and doing so will save you needing an extra reception-only enclosure card.