It looks like it's costing me about $15 / month. I'm using TwitterAPI dot io, which looks rather shady but has worked fine since I started using it in February. I'm following 84 accounts and ingested 14,554 tweets in the last month, which I think constitutes every tweet or reply by those 84 individuals and every reply to their tweets, although I may be misremembering exactly what instructions I gave. I re-check each account on a roughly hourly basis, a less-frequent check would be cheaper.