Lemons are available and cheap pretty much year-round in Southern California, but they're pretty expensive in December in Chicago. The best place to look is your local university extension, but here's some info to get you started. I worked in the produce industry for a number of years, so perhaps I can give you some insight into how the pricing of produce items works. The first rule of produce pricing is the bigger the supply, the lower the price. You should become aware of your local growing seasons.
Adverse weather can reduce supply of some crops and send prices soaring.
Broccoli and strawberries drown in heavy rains, for instance. Freezes in citrus areas can ruin entire crops of oranges. You know your favorites, just keep track and notice the trends. Interestingly, many fruits and vegetables are now available year round. This is due to the increase in imported produce.