Hi Clive,
If you looked long term and only bought the materials you need for the flies you fish then you'd probably make a long term saving over the cost of decent commercial flies but it would be slight.
Materials and prices for a basic fly like a hares ear spider are:
hook 20p each
bag of partridge £2.50 (will tie about 90 flies)
spool of thread £1.80 (will tie several hundred flies)
Hares mask £2.60 (will tie about 350 flies)
head cement £2.25
Thinners £1.80
Spool of wire £2.00
so knocking up a dozen spiders in different sizes is initially quite expensive but, if you add the following
Cobblers wax £1.20
Pearsalls #5 & #6a £3.20 ea
brass beads £2.20
Cock Pheasant tails £2.20
pack of grizzly genetic saddle hackles £12.00
Black & med olive dry fly dubbing £2.20
Pale olive thread £2.20
Poly yarn £2.20
Then you've also got Partridge & Yellow / orange, GRHE, GRHE gold head, hares ear dry, pheasant tail dry, Klinks, black dries - gnats, parachutes, midge. Pheasant tail spinners, olive patterns. In fact from the above list you could probably just about get an entire season of fishing on the river and rainbow waters, with another tenner or so and a selection of different hooks you certainly could.
If you stuck to that, you'd probably tie your money back in a few years but, the problem is it is all too easy to become addicted to this shit and buy materials simply because they look nice and may come in useful for something.
I work in a tackle shop and I compulsively buy every top (what I'd call 1st grade) orange or yellow bucktail that comes in, I reckon I must have about 10 of each still in packets. Then there's the stuff that you buy "cos is would make a nice x for y" that you never use because fly X catches anyway. by then you've accumulated so much that you can't find anything and have to buy something from the basics list above to tie more of the first pattern you ever attempted. Or maybe I just have a problem
Careful with road kill, you need to either chemically treat it, microwave it (very briefly) or deep freeze it to ensure it is free from nasties. I'd also recommend putting some napthalene crystals into the bag with any roadkill stuff just to be sure. In my experience, it is not really worth the hassle factor unless it is something either very hard to find or very expensive to buy.
Cheers
Ben