Immunising for Adults is a 2 jab deal and if you shop arround you might find it cheaper but to honest $81 isn't too bad. At this point theres not much you can do either way if he's been exposed he'll either get them or he won't, likewise for you. I'm a Nurse at a GP's and we don't check for chicken pox antibodies before we immunise. If you already have antibodies nothing will happen. I plan on immunising my kids against chicken pox...... when I have some lol
The best person to ask would be your mum (if you can) she'll know if you've had chicken pox, if you have you should be fine as will any unborn bubba.
Hope you doing ok.