If I buy a Rhino 7 for Windows license today, can I have a previous version on my computer under the same license?
Yup, You will need to get a legacy key.
Is there a cost difference?
No, If you purchase Rhino 7 (not upgrade) you can get a legacy key at no additional cost. If its Rhino 6 i would recommend keeping those on the Cloud Zoo. Rhino 5 isn’t account based and would be validated to the same machine as your Rhino 7 and would require you to be in one or the other, not both at the same time (since it’s 1 license)