The level of finish will determine how much you spend really.
Budget trike:
Donor Reliant -anything from the 10 quid I paid for my first one, to the 30 quid Ska Man paid recently for the one his engine came from, to 100 quid tops. Look for MOT failures. Check local free ads, Exchange & Mart, people's gardens. Look in places where they're not yet considered collectable. Reliant axles are starting to go for 100 quid plus on Ebay cos people have realised they're popular with trikers. We've actually got spare crankcases, heads, carbs etc, even a gearbox I think. If necessary, you could rebuild a good motor from a pile of assorted parts, (a nice little project for those long Winter evenings!).
2nd hand tank -I personally like Sportster tanks. Bog standard Harley ones are always on Ebay for 30 - 50 quid.
Front end -Depends who you know really. I paid 90 quid for the complete front end from a Suzuki Savage for my own trike.
Axle -Would hopefully come with your donor car, but they're about if you look.
Wheels -as flash or as cheap as you want to go really.
Seat -we go for the large solo pads sold by the likes of Highway Hawk. Several variations on sale from various universal custom part suppliers. Flap's & mine cost 40 quid each new, from Netley Marsh autojumble. In the shops they're about 60 - 70 now.
Frame tubing -we use heavyweight ERW. The cheapest stuff you can get, but it's fine for a well triangulated frame. Plenty strong enough. Enough to build a frame will set you back 100 - 200 quid, depending where you go for it -& how many mistakes you make!
The rest is all odds & ends you can pick up anywhere.
Rob seems to be doing pretty well with his build doesn't he. If he can do it, why not you?