The amount you pay for activating extra retainers is (#retainers) x (cost per month per retainer) x (#remaining months of subscription)
Currently I believe each extra retainer costs 1.40 euro per month. Two extra retainers would thus be 2.80 euro per month. If you have 3 months left on your subscription you would thus need to pay 3*2.80 = 8.40 euro to activate them for the remainder of the period.

Do these numbers look familiar?

For full information on hiring retainers see: https://eu.finalfantasyxiv.com/lodes...onal_retainer/