the m3mojo 15's subs require allot of air space for the to really come alive the very smallest ported enclosure i would put then in is 3cuft ~4cuft, if you dont have that amount of space easily available, but can be had by glasing the spare tire well, other areas of the trunk angeling panels and so forth. the more complicated the enclosure get ofcource, the higher the price of the enclosure is going to be. the materials, the design, in vehicle build and the labor, are all going to add up real quick, in conparison to a regular ported enclosure. $700 is not that much if they are going to do a good job to get the desired system goal to work.
making the best enclosure for subs that require allot of enclosure volume in a ported enclosure is not an easy task. if you wanted a cheap install. you could of gotten subs that required less air volume to play well.
allot of people buy subs just because of the brand, the look or hype related to that sub. and people lack to look at the subwoofer parameters to figure out if they can fit them subs in the vehicle. then are desapointed to hear that they not have the available space to make them sub perform how they where designed to, and that they have to compromise on the install.
youre either going to have to install the sub in an enclosure that is some what smaller then desired that will still get you a decent performance from the subs. or go bigger and install them in a combination of fiberglass and mdf enclosure using the most available space for the subs. but pay the price to get it done or do it your self.
here is enclosure Link to an enclosure i designed and built, for the same subs. its smaller then recomended but taken into account the cabin gain of the vehicle i managed to get a lage amount of low end bass at a very nice and loud levels out of them subs in a smaller then specified enclosure.