Typically outdoor patio speakers can last very long if taken care of. Since you don't have any coverage protection, it would be best to wrap them in plastic before any harsh weather conditions arise. You can find outdoor patio speakers for well under $500, but they will not be completely "water-proof". It takes a lot of work to make a speaker completely weather/water proof, and in turn, this raises the value/cost of the speaker.
Outdoor Speaker Depot has good quality weather resistant patio speakers ranging anywhere from $50 - $300. The nicer models have a port-hole design to give them an even fuller sound, but this creates more risk of water damage if not protected. Sometimes it's as simple as mounting the speakers horizontally and angling them downward so the water is less likely to seep into the grill holes.
Let me know if you have any questions at all.
- Brennan, OSD Audio