I cannot imagine that you'd be happy, long term, with a CNG bus...apologies to Bert who wants one! I just think that the limitations on fueling stations here in the US would be limiting...there are places you won't be able to explore. And I have no idea what the availability in Mexico is.
My suggestion, which has little value since I don't know what your plans or needs are, is to pass on the CNG bus...even at $3k it won't be a bargain. It's been said, and it's true...the cheapest purchase is often the bus itself...it's all the stuff which comes after that's expensive.
Having butted in and made a suggestion, let me try to redeem myself and find out more about your needs and plans. What kind of bus are you looking for? How big? Do you have a preference towards conventional or flat front? Front engine or rear engine? Will you be strictly highway and good roads, or would you hope to take your bus on forest service and dirt roads? Give us more info and the gang here can help guide you to some options.