Establish demand curve with infrequent sales
What you're looking for is the price elasticity of demand of your product.  That is: by how much will your sales rise or fall depending on how you manipulate price?  It depends on what you're selling and, like you said, each variation of your product will have a different elasticity.  So, say you increase prices by 5% and experience a %5 decrease in sales; your price elasticity of demand would be -1 (unit free).  

The strategy you outlined to parse out the price elasticity sounds reasonable from a data collection standpoint, albeit a bit risky.  I might change the price of just one of your variations (maybe one that sells somewhat more frequently)  to assess the sensitivity your customers are to price changes.  However, if your product's elasticity is high, your sales might drop dramatically or stop altogether.