$600 would cover about half the cost of the raw lexan for an all plexi bandpass enclosure. you'd need .75" to 1" lexan, and a 4x8' sheet of that stuff is over a grand. then you need to pay for someone to cut it, buff the cut edges, and assemble/cement/countersink and bolt together the enclosure all to spec for the subs you choose. you'll then need to pick subs that work well in a 4th order enclosure if that's the type of bandpass box you want. figure at least %250-350 per sub. then you'll need an amp for those.. so another $400-500.
why bandpass? i dont have a problem with bandpass, but it sounds like your building your system around the box...wrong way to go. for about $300-400 (on ebay at least), you can get a kicker s12l5 and a kx600series. thats a nice 600wrms of kickin power. just get a simple sealed enclosure for like $40. thats what id recommend...
simple but to specs. might be a little more than that. but why not try and make your own? mdf box is just as efficient for a hell of alot less. and deffinitely dont build your system around your box, build your box around your system. the end results will be much more satisfying. and as for subs id probably point you in the direction of alpine. people are probably going to jump all over me for saying that(except for 1 person on this site,lol and he knows who he is)but the type r's are one hell of a sub. especially if you are on a budget. as was i when i purchased my type r's a month or so ago, haha. but 2 type r 12's(1221d) and a mrd-m1001. over powering by a little but ive seen it done. i have my 2 12" type r's(1241d) with 2 mrd m501's and they take it very well....alpine mrd m1001-$445.00 www.ikesound.com 2 alpine 12" type r(1221d)-$195.00 buy it now at~~~>http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=18803%26item%3D5742043073 %26 andi bought all my stuff from those 2 exact places and it was all good. everything worked, everything was new, shipped on time, and easy to deal with people who ANSWER your questions. yeah...i know what your thinkin..and no, im not kidding lol. but anyways let me know what ya think and if i helped ya at all and remember to always stay open to other options. some other guy might come along tomorrow and point you in a better direction which would benefit you more. and also let me know what this would be going into, it might make a difference in what ya might wanna get.
Do you know why Glass recommends "high" end stuffs? Cause if you listen to one, you won't go back to the cheap stuffs. So if you can, save up, if not then there are other alternatives. You can't knock a guy for recommending good products.