Advice for replacement/repair of an HVAC with a cracked heat exchanger?
Check your unit's warranty. The heat exchangers on some units are warranteed for 20 years or longer. You may be able to get at least some money from the manufacturer, or a credit toward a new unit of the same brand. They may also cover all or part of the installation labor.
Find out whether there are any federal or state tax incentives that could help. A federal tax credit has been available in the last few years for improving your home's energy efficiency, and replacing an older heating unit with a higher efficiency model was one way you could qualify. I don't know if that's still available in 2012, but worth looking.
Your local utility may also offer some sort of incentive for installing a high efficiency unit (not to mention the lower monthly bills).
Some utilities have programs like this one, where you can save money on your electric bill by letting the utility reduce your energy usage during peak periods. (In simple language, that means they'll turn off your air conditioning for short periods on very hot days to help avoid brownouts.) This probably won't affect your choice of a new unit, but it's something that you might want to consider at the same time.