Xtol works well replenished and if you do a search you will find detailed instructions about how to do it. It's quite easy.

Kodak documentation states that Xtol will keep for 6 months. Some people have reported keeping the stock solution longer than that, but it does not change color with age and from I understand, when it quits, it quits. If you use constant replenishment you can keep the process going for years by adding fresh developer with every roll developed. It becomes very cheap if you do that.

DD-X does get rather expensive mixing it 1:4 as the instructions suggest and using it one time. A single batch mixed 1:4 can be used for multiple rolls, but Ilford states the quality may not be quite as good or something like that. Some people use DD-X at higher dilutions e.g. 1:7 or 1:9 one-shot and that helps to reduce the price per roll. I recently picked up a bottle of DD-X to try when my current batch of Xtol reaches the 6 month time at the end of this month.

Dave