On Jan 15-th I did the nice order for new version Fomatone 542 II Chamois 20x24 at Silverprint. Net value was GBP 37.44 / 10 sheets. I was lucky because few weeks later the prices for exactly the same 20x24 Fomatone 542 II changed . Net value at Silverprint is GBP 43.60 / 10 sheets now. It is little over 16% up since January, but the paper is not double priced.
The Chamois gives me a great d-max and the base is so beautiful - definitely my favorite finish worth the extra money.
Increases in energy and material costs are the main reason for these increases - Foma are just following the almost 20% rise that Harman has had to impose over the last year.
One factor that I don't think has ever been pointed out here is the nature of the distribution chain for photo papers. Harman hold very large stock of material in finished rolls, and can convert this very quickly so generally are able to supply most of their catalogue from stock.
All the other paper manufacturers, operate with an order/manufacture cycle, in the case of Foma it's monthly. Orders from the distributors have to be in by a certain cutoff date - paper is made to order and shipped about a month later. If a large order from a customer comes out of the blue after the monthly order is in production, just tough, there will be a month's wait before it can be supplied. It's unlikely to be on the shelf to airlift over. To make the process even more fraught, the distributor will usually have to pay upfront for some or all of the order.
So Harman have the whip hand in being generally always able to supply from stock (although of course they have to finance this). The other paper manufacturers in the frame can run a tight operation as they are insulated from stock holding, but in effect always have a 'brake' restraining their potential sales, as the stockholding is by distributors who operate 'just in time' which sometimes ends up as 'just too late'.
It's probably in recognition of this that companies like Foma and Adox are gaining an innovative edge by researching new products (532 ll & the drive to remake Polywarmtone being examples). As several people have pointed out, you're paying for quality and elitism, so bear in mind a difficult procurement process, and order well in advance!