I bought a second hand window AC a while back, a 1 ton Voltas for 6 k. I had to get it serviced once, which cost me a bit. The only problem was that it's cooling effeciency had decreased a bit and the power consumption was a bit on the higher side.
Still, like you I was living in a rented house and only used the AC for two months. Worked out perfect.I guess you could find one for around 5 to 6 k.