The 25+ domains utilizing MySQL are going to use the majority of RAM on your VPS. It will depend on the amount of usage/traffic there is on those sites. If you have very light usage/traffic then you may get away with something smaller, however, assuming moderate usage, I would recommend a package similar to the following:
384 (min) 512MB+ (preferred)
200-300GB Monthly Transfer
You'll probably be looking at plans in the neighborhood of 30-50 dollars depending upon if you decide to find a host with managed vps plans or unmanaged plans. There are also a lot of hosts who offer frequent deals and coupons so you are likely to get it cheaper if you find one of those "deals".
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