A wise bloke once told me "something is only worth what someone is willing to pay for it" and this is especially true for anything secondhand. Therefore what you or we think it's worth might not help much. If you put this on ebay you'll soon find out what it's worth - the concept of supply and demand will take it's course.
There are four older-geometry Rockhoppers (both steel and aluminium) on there right now with auction and buy-it-now prices between GBP30 and GBP155 but at the time of writing this there are no bids/sales on any of them:http://www.ebay.co.uk/itm/SPECIALIZED-R ... 4d0a6ab030http://www.ebay.co.uk/itm/1989-17-Speci ... 2c6d29aa7dhttp://www.ebay.co.uk/itm/specialized-r ... 257b9b08cbhttp://www.ebay.co.uk/itm/Specialized-R ... 3a7f57d322
Trying to sell a bike which cannot even be ridden will eliminate huge swathes of potential buyers because many people are not mechanically minded, don't want any hassle and just want to "buy-and-ride". Only nutcases like us lot on here spend more time tweaking bikes than riding them. My guess is that if you auction it in this condition it won't even reach GBP50. Try to fix it first if you can do so on the cheap, but don't throw much money at it, you won't see a return.