Now to answer your question...
You can buy an Enfield for $500-$600. Usually you have more leverage if you buy from a fellow traveller.
Expect to put dribbles of money towards repairs regardless if the bike is in great condition. We're not talking huge sums, a couple dollars here and there. I had to have piston replace and it cost me $25, but I was taking the bike home so I kept it up and did extra work on it.
You probably already know this, but in India travel with extra cables, spark plugs, tire tube, patches, and oddly enough a chain (mine fell off and before I could even stop to fetch it some boys snatched it).
When it came time to sell the bikes I didn't keep, I sold one to a fellow traveller and one to my brother who was arriing as I was leaving.
Travellers like buying from travellers because we tend to keep the bikes up better. When my mechanic opened up the bike to do repairs their were make-shift parts out of coke-cans, and other random bits. Ingenious and I admire their creativity. However, once I had all real parts put in it ran like a dream.
Any way, you shouldn't have a problem reselling at purchase price if you leave yourself enough time to get the word out and are in an area with other travellers.
Goa, Mumbai, Delhi and other areas are great for buying and selling. I bought one in Katmandu and one in Mysore, both from travellers but it was shere raw luck.
When are you in Mumbai? I'm hoping to rent and pop down to Goa for a bit. I usually travel in the north or in the Bangalore area and never took time to check out Goa. I'm hoping I'll be catching the end of the season, and not be too busy.
Good luck! Have fun!
Yoga Hoolagan On an Enfield