they are using one of the drugs moreso than another as a profit center.
IE, given no other constraints, I'd expect a pharma to distribute its R&D cost over its drug profit centers weighted by the drug's profit margin, and that all the drug's profit margins would be roughly equal: naively, I'd expect them to raise the price of Avastin and lower the price of Lucentis, assuming the costs of producing them truly are similar.
The reality is, likely there are proprietary reasons we're not privy to that causes the cost differential. They could be technical (maybe it does cause much, much more to produce Lucentis), or the could be business (maybe increasing the price of Avastin could cause it to sell poorly against another drug from a competitor)
How else are they going to fund future R&D?