If you don't work for several months at a time, then the rental-model license would seem to be ideal for you. The smallest increment is one month, which is reasonable. Short of cloud-hosted software I doubt you'll find hourly charge-out rates for licensed software. If you have about a month of work then you get the one month license; if you land something significant then a quarterly license may be more appropriate. With a fixed term available you can cast around for other work that can be fit into that timeframe. With intermittent work the rental model costs are in-line (or better, if less work is involved) with the return on a perpetual license with a much higher initial cost.
As for work thats measured in the hours (as opposed to weeks) you would probably be better off with one of the free non-AutoDesk clones. If you are tied to AutoDesk products ie a vertical such as ACA or a third-party utility, then your client charge-back will need to be adjusted to reflect current expenses - just like the rest of us.
----------------------------------
If you are going to fly by the seat of your pants, expect friction burns.
"I don't know" is the beginning of knowledge, not the end.