Personally... im not a fan of most things pay4plugin(tm).
But if you're going to do it, nothing we can do about it really, some customers maybe put off, others will stay.
I cant see myself going unless more plugins cost to buy than there are free, or they overtake the use of the main bot.
While on this topic, expanding on the forum reputation system could be an option.
Maybe users with a certain rep level could be rewarded with one of these new plugins without the cost, but those who are willing to just buy it on the spot can still access it.
This could promote a lot of quality posts, benefit the community, while also giving users a 2nd option for getting the plugins if they decided for whatever reason they dont wish to, or cannot pay for it?
And again donations are always an option without scaring away customers with the dreaded "pay for extra" thing going on.
But i trust in you saying there wont be many pay to use additions, only real major addons that took a lot of work and rightfully should be paid for doing.
(Off to London for the weekend so will check back monday)