The pricing model seems unsustainable from a long term point of view and I've asked the developer questions regarding a recurring revenue model but received not responses.
It certainly does look that way - a perpetual license, free features and free updates every week. My analysis is that a steady income of new clients, support charges and selling of modules is where the revenue comes from. Mix that with keeping overheads extremely low then the business model is plausible; if somewhat fragile if new business drys up. To be fair to KBKP, I don't think the pricing has changed in the last few months but it has changed considerably since May 2013.
Investing in a winning product is not a concern but gambling the future on an uncertain one is not something I'd be interested in.
I certainly cannot offer you any comfort on the future! As a very small business we do not have Procurement or Vendor Management departments who can manage, monitor and track foreign company health. I have called for KBKP to stabilise their pricing for some time and they do seem to be attempting to do this. Attempts to open communication channels for free bug support has improved - although it could improve more alongside just general contact. My overall observation is that they are trying to stabilise and have accepted that older clients have gone and won't come back so targeting new under their current business model.
When you say you have invested heavily in HostBill do you mean from a development/customization standpoint or developer support costs?
In 2013 we were looking to revamp our in-house developed client portal. When we looked at our competition, our offering was abysmal. When we worked out the cost of in-house development with a huge uplift in features, HostBill was a clear winner. We have spent considerable money on licenses, plugins, themes and support. However, that cost is still (even today) less then our in-house estimation. I think the combination of not living through the Terms and Condition changes of May 2013 and having a good budget has seen us with less problems/issues than many.
HostBill, thus far, has worked and continues to work for us. We have recently moved to the Blue Theme and migrating to new 'CompareStack' order pages. Have look at our site to view the 2015 refresh and get a feel of what you can buy either as a bolt on in entirety if you have enough budget.
Feel welcome to ask some more questions.