I make all my mounts and prep everything for inletting before I touch the stock. There is always lots of work to do to the lock with filing, polishing and tuning. There is usually similar work to be done to the set triggers if you use them. I can build an iron mounted rifle for the cost of a stock blank($80 buys a good one), a barrel ($300), and a lock ($150) with a touch hole liner and few screws thrown in at about $20. The iron for the mounts can come from my stock/scrap pile. So, if you wanted to stretch out the purchases, start with the mounts that can be made for free (theoretically). Then purchase the lock, which will keep you busy for a while; followed by the barrel. You can drawfile the barrel, shape the tang, and cut the dovetails using the tenons and sights you should have already made. Don't put the tenon in yet, though. That will make inletting the barrel harder. The last thing you will need is the stock blank. I can and have made many guns for that direct material cost. Set triggers will cost you about $50. Of course there is a LOT of learning that needs to go into making the mounts and other parts ostensibly for free, but you can do it all for $600-$650. You could reduce your barrel cost about $100 by going with a straight barrel but I wouldn't do that unless you are building a target rifle. It is not harder to inlet a swamped barrel than it is to inlet a straight barrel so don't let that worry you.
The fancy carved and engraved personal chunk gun I made cost less than that because I had had some of the parts a long time. I don't think the custom Getz barrel cost me $200. I probably paid $120 for the lock and I paid $60 for the stock blank. I know the price of the wood because it was written on the blank. The triggers where about $40. There was a White Lightning liner which probably cost less than $10 and an assortment of screws, also less than $10. The rest came from the scrap bin. I forgot the ramrod. I always bought them in bulk so it probably didn't cost more than $2-$3.
To go the low cost route, you probably will need to get an local (relatively) experienced builder to help you. The more you know, the less you have to buy and an experienced builder providing hand on help can impart that easily. Also, if you are working with another builder, he will probably throw some of the less expensive raw materials if you don't have them, not to mention provide forge facilities and other specialized tools. I would and have been willing to provide that help to anyone who showed up at my shop. Call or e-mail first though
