Options for monthly payments and annual payments on same contribution page
You could make a single priceset which is capable of accepting the various combinations that are possible, then implement a design which provides a different UI for selecting the preferred amounts.

An example CiviCRM membership/donate page which implements a similar approach is the one at http://greens.org.nz/join - the visible elements trigger changes in (hidden) CiviCRM form elements to deliver the required user experience.

CiviCRM permits you to ensure a minimum amount on the contribute form, but that won't account for the combination of payment term and amount. With a small extension you can implement any validation required to ensure the correct amounts are paid (so a user can't submit the form with a cheap annual membership).