Like mentioned above...
Why not ad a step at the end that check the price that will be placed in in the cart against the price in the account. That step/code section should never get modified and is a fail safe. And of course you wouldnt use the same varible as you did in the prior steps. That section of code is clean safe and does a direct check.
This way even if you guys choose not to test something that is so important before you install a new version then the price thing will never happen.
Also, that way you could have all the newbies you want to develop your code at 1/10th hourly rates, promises of free e-junkie service for 6 months and e-junkie t-shirts, yet still never have this problem.
By the way, e-junkie t-shirts are "Chick Magnets" for sure. Why not ad e-junkie bright orange thongs?