First off since this is probably one of your first pairs of DIY speakers you are best off going with a laid out plan rather than designing your own. To put it succintly, I think the pair of DIY speakers you are looking for, based on budget, criterion and like commercial speakers are the Modula MTM DIY monitors that Jon Marsh & Co designed over at htguide.com/forum. Look in Mission Possible DIY.
The ones you want are the update with the RS28a tweeter rather than the SEAS 27TFDC, which is also a great tweeter but not quite up to par with the now released RS28a. The xo's are costly, around $490/pr, but this is where the magic lies. The drivers aren't too expensive, but don't write that off, they are high performance, even more so than SEAS Excel series with the given xo. I commisioned John @ DIYbox.com to make me some cabinets, stand mounted although you can go floor mounted for even deeper bass, and Jon Marsh recommends doing so for music monitors. John is charging me ~$250 for a pair of custom made, stained to match furniture, real wood trimmed cabinets. IIRC, the drivers are in the $150-200/set. This is near your ballpark but probably will make speakers to enjoy for years without worries of upgrades.
Don't discount the xo that Jon designed for these speakers as it is the key to making the most of the good drivers and MTM layout. He is a very good DIY designer, who has been around a long time building speakers and has a reputation to back that up.
