Hi xtremeslide, I'm just down the road from Hammersmith in Ealing. Long way from Andalucia (wherever that is!).
The cheapness comes from biding my time. I got the steelie in bits from a mate of mine almost two years ago. It cost £270 and came with a box of spares, including a new output shaft. In the meantime I have a few other bikes dotted about; I've built/restored a couple of RD350s (YPVS and LC), an RD250LC, a KTM LC4 amongst others - all of which I picked up cheaply. I currently have a YPVS/Mito thing onn the go, a knackered old XT500 and my main event - an RG500 engine in an MV Agusta chassis.
I work on these bikes in parallel, when I get stuck for an expensive part on one, I move onto another bike until something comes up. While I was considering the steelie rebuild, a CR500AF came up for £1000. Sounded alright on the phone but being 500 miles away, we agreed a price and I had it shipped here unseen (from Scotland). It was a bit of a disappointment. The "conversion" involved cutting away the
bottom engine mount and drilling through the front mounts to fit the CR motor.This makes the motor too high, the exhaust simply won't fit, the cylinder head lifts the tank by an inch and the carb is miles from the airbox. Still its about the right money; an engine will set you back £5-600 and a chassis about the same. I'm afraid it's not a great example of how to do things on the cheap, not until I flog off any unwanted bits.
So I waited for a while and bought a bare frame for £75, that's the one in the pictures with the new brackets. Since then and as we speak, I am being offered a complete CR250 '97 rolling chassis for £300 odd quid. I am turning it down because next time I want to do a CRF (and the van has lost its clutch).
When it comes to the conversion process I am relying heavily on the information from this site. I thought it only polite to post here because this site hosts the drawings I am working from. I am a (very mature!) student and have access to enough machinery to have a crack at just about anything. It's a steep learning curve though and I am not doing alloy welding until I can afford a decent AC TiG. Currently I am learning DC TiG which I will need to complete the RG500 exhausts. I have to do it myself because to have a custom set made up would cost in excess of two grand! So I bought some part complete's off eBay which I should be able to modify without ruining them more than absolutely necessary...
That's how I do things on the cheap. Oh and I built that shed at the bottom of the garden to do it all in (on the cheap
)
Screwd.
$200 chassis, free conversion kit, etc. I was seriously like less than $1000 into the entire project 
goodluck mang and if you look around there are plenty of finished builds to look at for reference.