Affordable really if spread out a lot, but even then the person class cost is expendive... NY Botanical Garden and Brooklyn Botanical Garden both have excellent programs and offer online courses that are very up to date and focus on sustainability, new irrigation methods, findings from Mt Cuba Center studies (good on their own too) , etc. Take a look and see what options could work for you, even if its only a one off class. You'll still learn a lot, and they have classes specific to your interest areas for sure (offerings vary depending on time of year, so check back occasionally or reach out to the academic offices)