What aspect of the price is the problem? Is it the animal? The cage itself? Lights? Drippers or misters? Food? If you can't afford to set the cham up correctly you will end up with major health problems that will cost you big bucks in vet bills (pay now vs pay later). Maybe you should wait until you can gather the supplies together.
There is no real reason why you can't set a cham up in a tree, but there are some risks. If you have cats, dogs, other pets that could kill or injure it, if there are places in your house the cham could get lost in (or cold places the cham could wander into and get chilled, hot places to get burned such as electric baseboards), or even escape to outdoors through open doors or windows. If this is a baby cham you will have to be extremely careful not to lose it. The window will not provide much if any UVB as glass filters it all out. Correct lighting is not an option its required. You would also have to keep the entire room at the correct temp and humidity. I'm not saying you couldn't do it. I have free ranged chams and enjoyed it...you just have to design it well and be aware of all the hazards first. There is an article about free ranging in a past issue of the Chameleon Ezine (www.chameleonnews.com) that could give you ideas.