The retail price of RSLogix500 to use for programming the SLC500 is $1100US (unless it gone up recently), IF you know a source to obtain a legitimate copy for $150 then please let me know WHERE.
You cost will be $ 500 for the starter package from your dealer. Pay the price and do it right or they will catch you if you later call AB for any kind of tech support.....
P.S. Starter package does most of the stuff you'll need. One drawback is that you won't be able to do any online editing. i.e. you must take the PLC out of RUN mode to download program changes.
There are no low-cost third-party programming packages for the SLC500.
Only way to find some cheap software: ebay. Search for "RS Logix 500".
Be sure that you get a master disk with a working license.
AND, that the software is fully transferrable. If it is not, it is probably a bootleg copy and you wont be able to upgrade.