One formula to generate strong returns on equity markets involves investing small amounts of money in top stocks regularly over long periods. Buying fractional shares is one way to apply this strategy, but it's also possible to find attractive corporations at already-affordable prices.Here are two excellent examples in the healthcare industry: Pfizer (PFE 0.30%) and Exelixis (EXEL 3.72%). These drugmakers are worth investing in, and both are changing hands for under $50 apiece. Here is what investors need t ...