You don't need to display a radio button, you can use the number of employee field and use conditional statement on plan in a similar manner.

There is no way to validate the selection of a plan against a field value, but there is a way to not show the plan at all depending on a field value condition.

I still don't know of a way to dynamically change the price of a plan depending on a field value.

Followup full proposal based on link Beat has already given you.

The approach is as follows:

To conditionally show a specific plan during registration based on the value of another CB registration field, you can use the conditional constraints in the CBSubs plan (see the plan workflow tab and CBSubs manual for details). Idea is to show the plan only if the value of a number of employees field is within your specified ranges.

Now, regarding the "dynamic" pricing ...

Lets assume you want the plan price to be X for each employee (one employee minimum I assume).
You should set the initial price of the plan to X (to catch the minimum).
You should also not show this price in the plan (since it will not be the final price) - just set Price display of plan (default: [AUTOMATIC]): to blank or some other message (e.g. price will be calculated during checkout).

Then you need to create a promotion that will use an CB Query field to produce an SQL statement that will multiply the employee number times X to produce the extra pricing that should be added to the price plan.

This is the concept proposed.