First of all prices are relevant to many things...they are not calculated by the magical price fairy nor do they burst into existence in defiance of the laws of physics.
They come down to a few things including:
- Investments in time and resources for creation
- Future time projected for support/maintenance
- Consumer base exposure
- Marketing strategy
- Market value
The one to pay attention to here is consumer base, XF has the whole of the capable minded internet usership as potential customers, while an addon developer only has the potential to make sales to those who are already license holders of XenForo.
This drives the cost of their work up as the cost and profit margin needs to covered by a small slice of an already small slice of people and as Kevin pointed out this becomes visible because of the low price XF goes for.
At the end of the day if you want something completely custom that no one else can get, you are going to pay a ton of money.
If you commission something while paying less than rate and the rest of profit for the developer is going to be based on other sales of that product to the sub-usership that number has to be based on how much needs to be made and how many people are projected to buy it and not how much the original work you are modifying/adding on to costed you because that is how a developer needs to calculate things in order to run a sustainable model.
As far as sales, rebates, promos...some developers simply have the ability or the necessity to roll sales into their model.
It just is what it is, and it is business.