http://www.ebay.co.uk/itm/Mitsubishi...#ht_500wt_1156
The prices are all over the place i must admit! i probably would of stuck yours up roughly the same as you, really are so diverse! speaking to my evo breaking friend he sells 7/8 seats for around the ?200 mark ish! (condition dependent)
then made my subframes from ?20 worth of flat bar and ?10 of ZS seat runner from scrappy, can make anything fit anything
Obv not here to tell you that your wrong or whatever, more interested in helping you with a realistic price so you get some monies in your pocket asap