As already posted, the fee is waived if you join within a certain amount of time.
Also, keep in mind that the 1.5% of the base pay is the base pay of the old pay scale, not the B scale. You can get the difference rebated back to you.
That being said, if you're looking for an actual number I don't know if it's kosher to post it here...but, my night differential more than covers my dues - and I don't work in a facility that has mids. I have it deducted automatically.