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They are actually quite inexpensive.
You could operate them from a single virtual server - at least that is what I have done in the past. You could use just a hosting account, but there are several reasons that it wouldn't be an ideal platform for email - your best bet is to start off with a virtual server (I use GoDaddy, but there are many) and then you use the server as if it were your own. Get a single dedicated IP - you can authorize that IP in the spf records of all your domains. The best solution would be to separate domains by IP, but in this case, that would not be practical. I've used a single IP with many domains and made it work just fine.
Since all smtp email basically works the same way under the sheets, the software you use will depend entirely on what type of admin interface you need. Will they admin their own domains or will you admin all of it?
The pricing is there on the site.