This really depends on your sizes and how you want to display your pieces. If framing isn't an option, 3M spray adhesive and foam core is often a cheaper option, but it still isn't particularly cheap, particularly after mounting hooks and such are added in there. You can use an oversized foamcore board to also have it act as the photo matte as well.

If you will be doing a lot, getting dry-mount equipment can also be worth it in the long run, but the presses themselves are a bit expensive to get started. Again though, the prices and savings largely depend on the size you are working in.

Another option is to use cheap standard size frames and then use your own mattes to mount it appropriately. When you start doing multiple layers though, the dry mount capability becomes more and more important since it's easy to screw up using the spray adhesive and have to redo a mount.

Update: At those sizes, foamcore is still an option, but it would still be probably $20 - $25 per print just for the foamcore. Dry mounting would also just about be a requirement at the 15"x30" size unless you are really good with doing spray adhesive mounts. You might be able to buy bulk foamcore at that point and save a bit though. If you have a local shop that does dry-mounting, that would also save you the start up cost of buying a press.