Like most hobbies, gardening will expand to consume all the time and money you wish to commit to it. Luckily, there are some easy ways to not spend the Last of the Grocery Money on this hobby. The time problem I can’t help you with.
Be a plant whore. Be sure to tell your friends and neighbors that you are interested in any “spare” plants that have popped up in unwanted areas. I scored a bottlebrush buckeye that way this weekend!
Check out neighborhood plant sales. Some gardening clubs have annual sales with excellent prices. They are often listed in the garage sale section of the local newspaper.
Try the big box stores. Menard’s, Home Depot, etc.
Start from seed. I especially recommend this for the easier veggies (like cukes, squash, and beans) and herbs.
Cultivate patience, patience. Buy one hosta, wait several years, then viola! (heh) Two hostas!
If all else fails, with cover of darkness and a spade, you can save even more money. The neighbors probably won't even notice.