10 of the large instances costs $5000 a month. Not insignificant. Smaller ones in my experience don’t run larger Rails apps very well.
I've been in meetings where the combinded cost of the time taken to discuss if we should use a thing was more than the cost of the thing. Utterly infuriating.
Even when I worked for an agency there seemed to be an automatic discounting of the cost of people's time vs spending actual cash (and cashflow wasn't an issue).
I appreciate that there is often good reasons to DIY, but when there are not I will always favour something off-the shelf unless it is significantly more expensive.