I mostly make very, very, very hoppy beers and I always use extract and steep specialty grains.
My LHBS has LME for $2.10 a pound. I pretty much always do 7.5 pounds of LME so $15.75 per batch.
I get about 4 pounds of specialty grains to steep. At $1.50 a pound that's another $6.
I buy hops in bulk from Hopsdirect. My '09 shipment works out to about $0.80 an ounce of hops. I probably use 7 ounces in most of my beers, so $5.60.
I get my yeast for free from a local microbrewery. So $0
Not including the cost of priming sugar, equipment, water (tap), natural gas, burton salts, and bottle caps, that's just under $30.
Obviously, the biggest expense is the LME, especially since I'm using so much. I could probably knock off about $10 per batch if I used all grain.
For me, I make such a limited range of styles (blondes, APAs, IPAs, CDAs), I don't have the time for all-grain, I have a small house (less room for equipment), so extract brewing works for me.
If I had the time and room, I'd love to go all-grain. However, if I started making lagers or belgian style beers, I wouldn't just need to go all grain, I'd probably also need to BUY yeast and more equipment.
If you want to go all-grain to get a more thorough understanding of the brewing process and more satisfaction in brewing "from scratch," then do it. Homebrewing (whether mr. beer, extract, partial mash, or all-grain) is an exercise in futility. Every brewer is probably brewing beer that costs more per fluid ounce than most commercial beer, but is having lots of fun doing it.