so a condo can really just demand a lump sum of money on the spot?
A condo will generally have more money coming in through HOA fees so that some can be saved in order to pay for big ticket items (industrial water heater, roof, parking lot pavement, siding, you name it). They put that money in their reserves.
If the roof is fucked, or a pipe breaks, and the HOA needs to come up with more money to fix the broken stuff than they have in reserves, they'll need to resort to imposing a special assessment to cover it (or cover it and throw some extra into the reserve, or use some reserve and use the special assessment to cover the rest).
That's why ballparking it all is tricky, like I said, there's no way to look at the HOA fees to determine how solid an HOA is. An expensive HOA in a decrepit building might be scraping by, and a cheap HOA in a nice, well-maintained building might be flush with cash for when the shit hits the fan. The converse could be true too, a cheap HOA might get wiped out by something like a leaking roof, and then it's a special assessment.
That's why it's kinda hard to ballpark. Gotta figure out how well these HOAs are floating on top of figuring out how much the HOA fees are (you could jump in an HOA then get nailed by an assessment that averages out to a couple hundred bucks a month over the course of a year).
Most HOAs have enough money coming in that special assessments don't really happen, but looking at their reserves and previous special assessments and the condition of the building can give you a very good idea of what you should be paying per month, averaged out (the HOA fee, and if you think the HOA will run into trouble making ends meet).
Your agent should help you out with figuring all of this out, and most mortgage companies will scoff at giving you a mortgage if the HOA is in shambles (as they're out money if you default on a pile-of-shit-building), so you don't need to worry a whole lot about this. But it's something you need to gauge in order to snowball any perceived future special assessment into the HOA cost and average it out.