I can assure you that there are a LOT of variables:-
Just some:-
Paint - are you buying three rattle cans of do you already have a tin of chassis black you intend to brush on. What about primer / lacquer?
Are you using disc brakes? Do your donors have a matching pair or are you going to have to buy a matched pair? Did your donors come with worn out pads? Are you intending to upgrade to hydraulic?
Rim brakes - do you want new blocks or are you using the old ones that possibly don't match?
Is there a metal supplier near you or are you having to pay postage?
Are the headstock bearings your donors have knackered?
Are you using the cheap and nasty shifters your donors came with or do you intend to fork out for something better?
Are you using the derailleur your donor came with. It'll likely be cheap and nasty so you may want a better one.
Are you paying local shop prices for your cables or have you bought in bulk because you know you'll use them?
Do you have a matched pair of wheels? Do you care?
Can you find a nut for that bolt or will you have to buy one specially?
Are you following the plan exactly for the seat? Are you using some quality expensive foam or using a camping mat? Do you have an old leather jacket that's going to serve as the seat cover or do you have to buy some material? Are you splashing the cash on a fancy ready-made seat?
Do you have that tool you need to remove the freewheel?
Do you have the tool you need to remove a cartridge bottom bracket?
Are you making your own hubs or buying some?
Are you going the fasten those 4 mismatched old chains you have together for the job (you'll use 3 to 4 chains for a tadpole trike) or buy 4 new ones? Will you buy quality chain or cheap crap?
Do you have a nice set of perfectly shaped handlebars on a donor for under seat steering or will you weld something together or buy some.
There are cheap rod ends and good ones - which are you going for?
There are a lot more ways to spend or save. I'd suggest that if you have a decent workshop and are thrifty and ingenious you could make one for Sub £100. It'll repurpose most of the donor parts, only buying new where you have to and you'll need to scrounge that bit of wood for the seat and sacrifice that camping mat in the attic for it's foam. The fasteners you'll need you'll already have because you're that DIY type of person etc etc. Equally you could spend £700-£800 on one with new quality shifters and chains etc and an impressive ready made seat.
As examples if you look on
this thread the red trike was about £300 and the black one about £350 to build. That doesn't include consumables such as cutting discs and flap wheels, nor welding wire and CO2.