I think mine wound up to $145 or so, considering I bought some things when I first started that I wound up not using.
The pricing goes loosely like this-
A figure will typically run about $10. If the figure you're looking for is more rare, it can be more like $20, but you can probably find something to make a satisfactory Burt for within that price range. Sadly, as last I checked the particular SOTW head on my and ActionBurt's figures is a little rare, but Brian managed to find a good substitute IIRC.
You might wish to get one figure, but then also get a loose head, or swap heads from another figure. Loose heads tend to be $3-$6, where of course if you're swapping from one figure to another, you pay the price of two figures.
Clothes run the gamut for prices, but you can on average get a good BDU set (jacket and pants) for about $10. T-shirts tend to be around $5, snap-up long-sleeved shirts tend to float around for about $8. Pants on their own tend to be $5-$10. Vests tend to be $5-$12, although you can get really cheap ones with non-working plastic pockets also. Hats tend to be a little hard to find loose, fluctuating around $1-$4 when you see them. I'm still using hats stolen from other figures because I never actually spent the money for better ones loose, but I found a few when I looked long enough. I think the "Raider" figure hats are cloth, or at least made more realistically than the Joe plastic caps. Oh, and if you care like me, socks are available for about $.30 a pair usually on War-Toys ^^ Half-gloves are similar, but are a little hard to find. I got mine for free from ActionBurt, so I'm a lucky case. But I'm pretty sure if you do find a site that carries them, they tend to have a ton. Go for black ones if you can, not green- 21st Century Toys green gloves tend to stain the hands permanently. Although that's not so bad if you keep them on permanently.
Watches, grenades, flashlights, canteens, and other such accesories tend to be available on their own from sites like War-Toys for under $5 a piece (common dragon watches, L-bend flashlights, and canteens all tend to be around $.50!). Occasionally you'll see something higher, like a multiple-item set. MRE sets and things along those lines are more rare, running more like $10-$15.
Guns vary. A Barrett .50 will usually be $10, give or take 3 dollars. Handguns tend to be pretty cheap ($5 for the most expensive ones) and M-16s are usually about the same and are extremely easy to find. AK-47s are much like M-16s in this regard, but a little more rare.
Plastic Boots are pretty cheap, about $2 or $3 for a loose pair usually. If you want fancy lace-ups, prepare to pay $10.
I might've forgotten a major category somewhere, but you get the general idea ^^
The creativity comes mostly in trying to fashion your own custom parts- perfecting the face, making a hat like you want. That, and in selecting your parts, as there are a lot of options if you can find them (a plethora of gun types, different clothes, different heads and bodies).
I'd say if you want to make a really spiffy Burt and have $150, a week or two, and know the right websites, you could probably make a satisfactory Burt. As it is I never wanted to spend all at once, had trouble finding stuff, and a lot else going on with my time, so I took the greater part of a year with mine.
If you want to make a casual Burt, you can probably get a good one for under $60 and make it in a few hours time (once the parts arrive, of course). Open all your stuff, lay it out. Stick a logo on a hat with printed paper and glue or more complex means, or just use a plain one, paint the moustache and sideburns (by marker or by acrylic, whatever you find comfortable- I liked acrylic because I could chip it off if I goofed up, and it looks really good when finished you get it right). Put the clothes on. Done. ^.^
It's a great hobby, a lot of fun, and I really enjoyed it. A good amount of creativity is involved, but there's also quite a few degrees of accounting and searching for parts involved. I really feel kind of sorry that I've done so little on it recently, I've been pretty distracted with other things in life (and yes, this is the same reason I've been absentee at these forums) and have not had the time nor money for Burt. :( I'm trying to get some stuff going again in the next six months or so.