There is a new version of prestashop, that although i haven't been brave enough to install it, doesn't mean you shouldn't
Anyway and maybe i'm recapping stuff said earlier but:
a) Your theme should be compatible with prestashop 1.5. I tried multiple themes, and they all perhaps have code mostly designed for 1.4 prestashop (guessing here). The theme for some reason has the code responsible for pricing and showing sales. The theme i had most success was ..google thgr00027o, or graphileom. This isn't an endorsement, just saying sales prices worked -better- and is worth a try just to see what happens.
Create a customer account (with address) and view your shop that way! Without logging in, i see 'guessed' prices including taxes. Logged in, i see correct prices.
Hopefully this helps, after this i got nothin!