I don't know about half the price - my company (Zill.net) offers hosting from $19.95 per month.
A default OpenACS (depending on config) takes about 40MB RAM for the AOLserver process, plus a few MB per Postgres connection (although a lot of RAM is shared between the Postgres connections).
Busy sites (10GB traffic per month or more) usually take a lot more, up to 128MB per AOLserver process, plus Postgres connection overhead. Of course RAM usage is dependant on how much you cache, how much stack you use, number of threads etc. etc.
