I'd never purchase any single plug-in that costs more than Woltlab Burning Board 3 at its most basic commercial license.
WBB3 at its most cheapest is:
1 year update (no support) for 39.99 EUR / 49.99 USD
Anything higher than that is a rip off in my honest opinion, especially ones that don't even come close to taking as long as WBB3's development time and because it has no free counter part for people to test out, it certainly wouldn't be worth the risk of buying. Times are tough in this horrible economy, so I don't blame anyone who isn't willing to pay 130 EUR or roughly 168.05 USD (168.04551 at the time of posting). As for the 30 EUR, its roughly 38.78 USD (it's 38.77973 at the time of posting). I'd save up for rMarketplace if nothing cheaper shows up by the time you reach a saved total of 40 USD (to allow for exchange rate changes).
For 168.05 USD I could feed myself 157.06 (168.05 / 1.07 = 157.05607476635514018691588785047) cheeseburgers from McDonalds. That's 52.35 days with 1 cheeseburger worth 1 USD plus 7 cents tax per meal, on the basis of 3 meals per day.
For 168.05 USD I could get 4 years of updates without any support OR 2 years of updates and standard support OR 1 year of updates with professional support. If a single plugin purchase costs this much, its way too expensive.
The cheaper the add-on the more wide of a range of users that will be willing to part with the money. Being cheaper doesn't mean its poor quality; just like being more expensive doesn't make it better quality. And because there's no free-version of the Customer Area plugin to test out, I wouldn't risk 168.05 USD for a plug-in.
I don't believe 20 USD is enough for any shop system
20 USD is enough for a shop system. If I knew how to make a plug-in, I'd make a shop-type plug-in for 20 USD. Why? Because I'd be cheaper than the competition; I would also be targeting a wider range of users; Because it's cheaper, more people are likely to purchase my plug-in. The more purchases I get the more money I make versus the competition that's struggling to get more purchases.
"Customer Area" plug-in costs 129.99 EUR and has 58 downloads. Presuming those downloads are unique (58 different people) the plug-in made the author "viecode" 7,539.42 EUR
"rMarketplace" plug-in costs 29.99 EUR and has 396 downloads. Presuming those downloads are unique (396 different people) the plug-in made the author "Andoca" 11,876.04 EUR, which made 4,336.62 EUR more than "viecode."
If I sold a plug-in that rivals "Customer Area" for 20 USD or 15.47 EUR, I bet my plug-in would reach even more downloads than "rMarketplace's" total downloads. Why? Because my plug-in was cheaper than the others.
So why is selling a "shop" plug-in at 20 USD or 15.47 EUR not enough?