I just upgraded the parts on my '90 Trek 7000. I used a mix of XT and plain Deore parts, going with mostly the latest. I didn't do a fork nor a crank, and those 2 can start to cost. I'm about $500 in at this point, with dynamo wheels, XT v-brakes, Deore levers and shifters, XT M786 rear derailleur, chain, cassette. The dynamo wheel and light cost around $250 on their own, so the rest is another 250.

So add fork, crank, BB, bars, stem, seatpost, saddle. That'll eat up another $700 or so for decent parts. You could go the China route and order carbon Fork/Bars/Stem/Seat Post as a unit as I've seen advertised. It looks like nice stuff and it costs a fraction of brand name gear. For BB/Cranks I'd stick with Shimano, XT if you can find it cheap. Everything I bought at rock bottom prices from eBay, Amazon, Jenson, Backcountry, Cambria, UniversalCycles. Watch for free shipping as that can start to add up.

Excel will be your friend. Create a worksheet of what you can find at the lowest prices including shipping. Add it all up. Make sure when you buy shifters/levers it includes the cables, make sure your brakes include pads, make sure your hubs include the skewers.

If you go the Chinese parts route, take a look at the carbon frames too, they are pretty cheap.