£100 isn't a lot for an Airsporter, the parts that are missing or broken can easily be sourced and shouldn't cost a lot second hand. It's difficult to give valuations based simply on description only (pictures are better) as one persons description of good condition is anothers description of poor.
£30 is too low - you would get far more than that if you sold it off in parts.
£50 is very low for a working Mk2 - it must be pretty rough, I have seen the stocks alone sell for that or more!

ASM