Older model air conditioning units use R-22 coolant. These units are being phased out in favor of R-410A coolant, which is more environmentally friendly. This means that the old coolant will now cost about triple what it used to, if you can find it at all.
If you have recently replaced your air conditioner, you are in good shape! If not, the cost of buying the old coolant, and difficulty obtaining it over time, are going to add up. Also, forget trying to sell your place with the old unit. The buyers may have you put in a new unit before the sale.