I recall running LAMP stacks on something like 128MB about 20 years ago and not really having problems with memory. Most current website backends are not really much more complicated than they were back then if you don't haul in bloat.
I don't know what you value your time or opportunity cost as... but the $10/mo doesn't need to save very many minutes of your time deferring dealing with a resource constraint or add too much reliability to pay off.
If resource limitations end up upsetting one end user, that costs more than $10.
And most VPSs allow increasing memory with a click of a button and a reboot.
$100 is peanuts to most businesses, of course. But even so, I'd rather spend it on fixing an actual bottleneck.
For example: Ticketmaster makes a ton of money and their site is complete dogshit.
if the scalability is in the number of "zero cost" projects to start, then 5 vs 15 is a 3x factor.