1. Spend your time selling another copy to someone else.
2. Offer a monthly retainer contract or a case-by-case basis where they pay a minimum per incident, plus hourly fee if beyond 2 hours onsite. Or offer a discounted remote fix solution where they let you log in remotely, saves you time too.
3. If it is a complicated system, they would rather hire you than get someone else to dive in and wreck their system. Let them know it will cost extra thereafter for you to fix that guy's mistakes, and there is no more warranty if others touch your code.
4. Far more likely that a hardware, purchasing policy or other problem will make them stop using your system than that the system doesn't work or maintenance is expensive. For example I installed two turnkey search servers for a multinational, and serviced for 5 years plus added additional functions a few times. But maintenance done by an employee onsite, in fact I spent a lot of time on extensive manuals. I beat out Altavista and other solutions when I sold my system to them, but they stopped using it due to 1) reseller not supporting, and especially 2) lack of replacement RAID drives from a linux hardware manufacturer that stopped supporting their own system. There is also I think a 3) lack of reseller selling to other clients, so it cost me a lot to invent things for only one client. Even though it worked great.. a gigabyte comprised of 60 databases, searched with mod_perl and htdig in .1 seconds per query.
A superior solution to what they have now too, the missing ingredient is people enthusiastic to support the project either with sales or with hardware replacements. I think there is a lesson here for you too. If you have a good product get a steady list of clients who want to use you, offer additional development training and product updates, and stop thinking about obfuscation. Write some manuals and build a dialogue with the people in the customer organizations. Talk to the engineer using it and make them pay for you to come back and train people, etc.
My guess is an ERP system is complex enough that no sane client will think of bringing someone else in to service it. They will just throw your obfuscated garbage out the window and get a new one. While a client will usually not demand source code, they will be pissed if they have someone who finds out you are obfuscating. I've always offered manuals and full source code and you know what? They respect that and they also keep coming back.

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