The short answer is that a rule of thumb is 3x peak annual revenue, then discounted for risk, cost to bring to market, etc. Some would say a range of 2x-5x is reasonable. A quick look at any recent biotech acquisitions shows this rule of thumb to be just that, and not a particularly accurate one. However, it probably puts you squarely in the middle of a big ballpark of potential figures.
As was already mentioned, even cutting these projections in half results in a starting point market valuation of a big, big number. Even for combo alone.