The urn enhancer is an item that can be manufactured using the Invention skill. It requires level 24 Invention to discover. The blueprint used to discover how to make an urn enhancer is a reward from Nomad's Elegy. While the enhancer is in the inventory, each urn teleported grants 25% extra experience at a cost of 20 charges.
Each divine charge provides enough charge to enhance 150 urns, which gives an average cost of 477.3 coins per urn when charges are bought from the Grand Exchange.
When using the Urn enhancer, one must consider the type of urns used. Low xp urns are rarely worthwhile. This particularly affects smithing urns, since their strong variant just yields 250 experience. Generally speaking a point of experience is worth roughly 10gp — this would mean one should use the enhancer on any urn yielding more than 50 additional experience. This equals 250 experience per urn. One must conclude that it's always viable to use the urn enhancer, as Crafting level 80 is a requirement to obtaining it and therefore any urns should be of a high enough level.