Asking how much is like asking how long is a bit of string. I don't mean to sound facetious but I know guys who have built car based Trikes for a couple of hundred quid. I bought mine more or less as is for £4000 and it came with £5500 worth of receipts! I've spent at least another £4000 on it and it doesn't look any different, but it is a tad more reliable now (that's pushing my luck ain't it?)

There are so many ways of doing the job. Mine has a custom made rigid frame. I enquired after a new independent suspension rolling chassis and was quoted £6500 (no thats not a typo 6-1/2 grand) If you get someone else to do the work it'll cost an arm and both legs!! If you're handy with a hacksaw and welder get a shaft drive bike a Ford axle and some box section and then you can do it really cheap (but what will it look like?) Also registering the beast may be a problem. Using Reliant registration documents is one answer I've come across (A Ford 1500cc engined Reliant with no bodywork, Oh yeah!).
You can get conversions that bolt in where the swingarm goes. I've heard they cost from £3,000 or so, pictured below are American units for Harleys and Goldwings

If you are interested in building a VW based Trike then go to Slacker G's site here or Tony Alsop's site here for excellent "how to" articles.

If you buy something that someone (non-professional) else has built, be VERY careful, I wasn't, I just fell in love with my Trike and shelled out the dosh. Since then I've been putting things right that the bloke that built it cocked up! It was already registered so that was one less problem (about the only one).

The choice is up to you. My preference is for bike engines, but that's purely a personal opinion, I've seen great looking car engined Trikes and bike Trikes that look like a shed on wheels.
Car engines are cheap (especially the spares) and easy to come by, but you'll probaly have to build the frame from scratch. At least the engine/gearbox/axle will fit together. Bikes have the advantage of usually coming with a frame you can make partial use of (along with forks, wheels etc.) Another problem is the chain drive. You either have to convert an axle to chain drive, it can be a problem getting the right size of sprocket to fit in the diff casing (or the gearing goes all to cock). The other option is to convert to shaft drive as on my Trike with all the attendant problems of knackering chains at an alarming rate (coz it's so short it goes round 3 times as often as a normal bike chain hence 3 time the wear rate). I would suggest starting with a big jap shaft drive (Yam XS1100 springs to mind but they're probably a bit long in the tooth these days, a sign of my old age I'm afraid) and then some sort of swinging arm to mount an axle on. A mate of mine has this sort of setup on a Goldwing (see Dick's Trike on my Other Trikes page), he found he needed to run a Panhard rod, otherwise the handling was abysmal.
I once saw a Yam XS850 done with a four link set up and a very low seat, and it looked really good.
Go to some Bike shows take lots of piccys and steal the best ideas.

Insurance!! mine costs £250 fully comp but a mate of mine pays £75 for his Chevette engined trike (mind he's bloody ancient even compared to me)
I've been asked about insurance several times, try my broker, Whittlsey Insurance Services Ltd. Tel 01733 208117.

No, but having said that some of the boys in blue don't seem to appreciate this. It may be worth wearing one to save being constantly "tugged" (once they've stopped you they will either regale you with tales of the 1903 Wartburg & Gobdog they used to ride, or they'll go over your pride and joy with a fine tooth comb and probably find something they don't like. "This number plate is a half inch too small"). Personally I've been "tugged" once by a Wartburg & Gobdog type and he didn't notice the lack of speedo cable, indicators or silencers (all sorted now Honest!!)