Just to add: I have around 370 names listed, with 14 sales. I could look up the exact figures, but ball-park for now - my costs are around $40 per name, and I'd imagine I've netted around $1500 per sale. So my revenue is ~$16k with costs of around $14.8k.
Even if your cost of picking up names is $10, together with listing fee you're spending $20. Given an average sales rate of say 5% you'll need to list 20 names to get a single sale. If your acceptance rate is 50% then you're talking 40 names for a single sale. That's costing you $600, and the average sale is probably netting sellers around $1300 (($2k - 30%)-$100). Still a good ROI, but it's far better to consider your total costs rather than the cost of the single name you sell.
Add to the above the fact that 5% is probably on the high side based on evidence from many sellers here. There are a couple of users with 300+ names and 0, 1 or 2 sales. 300 names @$20 each is $6,000. Will take quite a few sales to even break even, and right now many of us are still trying to figure out a realistic sales rate.
Thankfully I'm selling quite a few domains outside BB, and being honest I'm very much in hold-mode trying to determine whether adding considerable more inventory to BB makes economic sense. I'm still positive on BB, but I think it's really important not to put all your domain eggs into one basket and to very carefully analyse your costs. Far too easy to focus on the big ROI sale and ignore all the flops that are required to get that one big sale.
[Edit: forgot to consider the acceptance rate on BB, and corrected the cost of 40 names
]