I do not like any commercially available forum software (which is why a2k uses custom-made software) but the most popular ones have historically been vBulletin (commercial) and phpBB (open source).
Wordpress makes good blog software and they made some forum software with an emphasis on simplicity, while I haven't used them much I had a good first impression of their software (BBPress).
As for Horizontal Verticals' costs I don't do the finances but guess that the costs are currently (we are growing) somewhere between $30,000 and $40,000 a month but I'm not sure what you are really trying to figure out as HV is the parent company for all the companies I have planned or founded and HV's revenue and costs mostly have nothing to do with a2k (which would be something like 4% of HV total revenue). If you are asking what the costs for a2k would be the fixed hosting costs would run you around $6,000 - $12,000 per year but human costs are always much more expensive and I don't have an estimate for the total human costs to run a2k as most of the time has been volunteered or shared across multiple projects and how you structure your site's HR resources can make a huge difference but for a quick idea the first version of custom a2k forum software cost me about $30k out of pocket for part-time programming bandwidth over the course of a year as well as a lot of my own time (which I didn't pay myself for). Obviously, running it on out-of-box software is thusly much much
less expensive (I would estimate that we've spent a couple hundred thousand dollars of man-hours on a2k programming vs a couple hundred bucks for commercially-available software).
So the bottom line is:
Don't go with custom software unless you have access to cheap programming resources (e.g. you are a programmer yourself and have spare time) or are seriously in for the long haul. The fixed hardware/hosting costs are going to be $100-$500 for 99% of forums but forums usually use a lot of man-hours to maintain and usually count on them either being volunteered for viability.
The real cost is time. Figure out where that is coming from and the rest is pocket-change in comparison.