Conditions support full substitutions (see info directly next to input for details and special functions). You could for example use the below.

Left: [cb_myfield]
Operator: Not Equal To
Right: not payed

But this action will send a mail EVERY time the profile is changed?

Yes, if that condition is met it will.

If you want it to send only 1 time when the field is changed you could use the below (requires 2 conditions)

Condition 1
Left: [cb_myfield]
Operator: Not Equal To
Right: [var4_cb_myfield]

Condition 2
Left: [cb_myfield]
Operator: Not Equal To
Right: not payed

Note in the above the "var4". The 4th variable provided by these triggers is the previous values. This means [cb_myfield] will be what I just change the field to and [var4_cb_myfield] will be what I just changed the field from.

The conditions are not what I supplied, please see my previous reply. The first condition needs to first check if the field was even changed. That's done with the below.

[cb_betalt] Not Equal To [var4_cb_betalt]

If the above passes it means the fields value differs from the previous meaning it was changed. After that you can do your matching for a value such as the below.

[cb_betalt] Does Contain 2012