Hi Kelly,
Yes you do need a boost - your result in unequivicol. Which means that you may have enough immunity or you may not. So, the decision is yours. It would be very stressful however if you came in contact with rubella and were not certain of your immunity status.

TTC prior to the 3 month recommended time may result in damage to your baby similar to that if you were infected by the virus.

I am not sure on the cost of the booster...