hi mate, well i have looked in the past for a seat replacement, and your talking thousand at least from the stealers, the come in halfs, and i dont think the headrests come with them either! dont forget there are side airbags in the front seats and they are expensive for that reason too.
what youve beenofered seems a bit high, but if they were tip top with no creases id go for it, if they are looking a bit tatty, hold back, there are usually some on ebay, a set the other week went for 350.
hope this helps.
p.s does the offer include leather door cars? as you would probably want these to finish it off