Hi Miranda,
Sorry I can't help you with potency levels home-grown vs. ready-packaged, but I can't imagine that there will be much of a difference.
As others have already mentioned, spearmint is very invasive and if grown outdoors, it should be confined to a container or pot. In the UK, all the big supermarkets sell growing spearmint and cut bunches for not a lot - what you should be looking for will be labeled as "Mint" as in mint sauce, rather than Spearmint. If in doubt, just pluck a little bit of leaf and rub it between your fingers before smelling / tasting it.
Also, if you live near a big city or large town, there will probably be an Asian quarter with lots of Asian fresh food stores - it will be cheaper from there and probably available in larger quantities than the small bunches or packs from a supermarket. If in doubt, you want the type of mint which is used widely in teas in North African countries, such as Morocco, Tunisia and Egypt.
Actually, whilst we're on the subject, for those of you who take FG, you will find it in these Asian shops much cheaper than in capsule form or from a supermarket. Just boil the seeds in water for 5-10 mins, then allow to stand for three or four hours. I used to use between one and two Tbsp per mug of FG tea. Protip: In Asian stores, they might know it better as Methi Seeds.
I seem to recall Abidrew85 giving instructions on how to make fresh Spearmint tea, and I could be wrong but I'm sure she said to rapid boil the leaves in water for ten minutes before drinking. A search on her posts will no doubt point you in the right direction.
Good Luck!
Ellie.