I got the seats used for $650/pair, they came with a bunch of the aluminum side brackets, from a guy with an E36M3 racecar. They are really too big for me (EVOIIs), but just fine for farting around on the street with.
The bases cost ALOT to get here. They are made by BRIDE, and I had them shipped here through a friend's friend's shop in Japan. The bases themselves were only a couole hundred, but the shipping cost me almost that much over again, due to bad communication.
Then, I had to do some cutting welding on the bases to spread the uprights to fit the EVO IIs. The Uprights on the bases/sliders now are actually a set of MOMO steel sidemounts, welded to the BRIDE sliders.
I have about $500 alone into the bases, but they are rock solid and slide fore and aft with NO, NONE, NADA slop. Slop is bad bad bad. Ask anybody who has a set of the W3dge Engineering brackets (I have one collecting dust in my garage).
also, the EVO IIs are wide, and when centered nicely, the outside shoulder wing contacts the door trim, and requires a firm door closing.