Basically if they buy:
1 - $149
2 - $139
3 - $129
4 - $119
Where would I set these values? I've read about pricing tiers and quantity pricing but I can't find it. I'm on Joomla 2.5 and Virtuemart 2.0.6
Upgrade you VirtueMart version. Multiple pricing is included in current versions. In any case, VM versions below 2.0.10 are considered as unsecure, as are Joomla versions below 2.5.9
If you use template overrides then you may need to edit those for the current versions. Update information available at http://virtuemart.net/index.php?option=com_content&id=416
The only problem with upgrading is I'm afraid my payment gate way (Linkpoint / First Data) will break from OSDCS.
http://www.joomla-payment-gateways.osdcs.com/index.php?option=com_digistore&controller=digistoreProducts&task=view&pid=162&cid=9&Itemid=5
The just test the update on a backup copy of your site and check the plugin works. You cannot carry on using a store site known to be insecure.