As stated, when a complete billing and support system is $18/mo and plug-in cost you $39/mo, it simply makes me appreciate Matt and his pricing even more. You are free to charge what you want. You developed it - you price it. It isn't going to change my opinion.
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I think Matt should be charging to advertise these modules on his forums. Matt could make some money here since you are basically given free access to beta testers and free marketing to your client base you sell to. But I do not run the show here either. But using your logic on pricing, being charged to sell here should not bother you? No need to answer, just having fun.