[GrinningFool]

It may make sense to expose a separate entry to retrieve the count. That way the 98% of users who don't need it can continue to ignore it, but you won't force the extra overhead of requiring the other 2% to page through all results to get the count.

While count is expensive, it seems it would be less so than full result set retrieval broken up over several requests?

You'll get some additional usage beyond the 2% who need it now (after all, if it's there people will use it while in its absence it would not be considered at all), but on the whole you should be able to find the threshold at which you'd come out ahead by offering it.