UK based... but that doesn't mean that they are required to participate in those sales. The price (new license) is already very reasonable for what you get and has not increased since I started using it in 5 years (2102) if I remember correctly.
All of the paid scripts cost is "huge" in comparison to free offers.
The only one cheaper than XF is WBB. I've used all 3 (WBB, IPS and XenForo). WBB is (for me) lacking a quality that I can't really put my finger on but I just never liked. Another issue with WBB is that their main level of support (user and staff) is in German. At times getting help can take much longer on the forum.
IPS is much more expensive with the initial price and yearly continuing renewal which you pretty much have to maintain to participate in their support forum ecosphere (unlike here at XenForo).
Trust me it's worth every penny we tried very platform from free to paid they all sucked then a friend pointed me to xenforo and we've been using it ever since save up money if you have to like stated by @Tracy Perry IPS is expensive at first purchase just what we needed was going to cost $275 because we needed gallery as well and they charge for their core
I think IPS is running a special for new purchases. Their forum deal gets you down to about where XF is. By default the forums (along with the mandatory core suite) is $200. I think it still ends up more expensive than XenForo for initial purchase price (around $170-180) and the yearly support is more than XenForo is.
You found one already! The normal price of XF is already a steal. Other then when RM was first releasing, I don't recall XenForo ever having a sale on any product. The price is already too low for the quality of the product you receive.
Getting back to the yearly support... with XenForo it's $70 annually for my XF site (no longer active) that was comparable to my IPS site I currently have active (which is $150 annually).
As most have said, bang for the buck XenForo already offers you a great deal.
The support (forum) related is much better than you get with IPS or WBB.
Ticket support is generally faster with XF compared to IPS or WBB (in my experience)
Level of fellow forum owner interaction/support is higher with XF than IPS or WBB.