The voucher should be good. I'm guessing you have entered in the voucher id when it asks for it, and you are getting an error message?
Without posting the actual voucher ID, or any other personal info, could you tell us exactly the error you are getting? I'm thinking the $150 statement may just be informational, and not an actual error message. Correct?
I'd say go through all the steps on the prometric site, including the picking of the location and exam time. You should then be asked for your voucher number, and when you provide it, the voucher should be valid, regardless of the suggested price.
Good luck. Hopefully the actual exam will be much easier than the registration.