yup pritty much every tinkerer ever has built a cocker, the good news is they are fairly easy to get parts for and the budget can be as little or large as you like depending on how crazy you want to get, from a simple mechanical cocker all the way up to an ego framed cut down sled-bodied mq-ed e2-ed crazy beast. or go old school and do a sniper (pump action cocker)
the sky really is the limit....enjoy!
last one i did (6 months ago) was a pump - total spent was around 450 (could be done a lot cheaper using a 2nd hand gun these days probably less than 300), i used an ebladed 2k4 eclipse cocker which i bought a couple of years earlier and took parts off and basically traded them for the pump parts, added a few nice touches and a very slick anno job (white with polished green) and im very happy with it.
before that i did the first turtle cocker in the uk (around 3 years ago) with the intention of making it as light as possible, built up entierly from parts, full eclipse internals, eblade, all in polished black, wieghed 1g less than a nexus cocker, lol. i think i spent around 500-600 again on that one and sold it for slight profit if i remember correctly.....it changed hands a couple of times since but i have seen it being used !
pic is a bit dark but
http://homepage.ntlworld.com/matthew.stockdale/Tur 2.jpg
before that i built my first every cocker (and first semi) around 10 years ago...one of the early minicockers which i added to bit by bit over a few years and finally sold it to finance another gun i was building, last time i saw it it had had a race conversion and looked pritty damn good for a 10 year old gun!