Short answer: Whatever the DM says it costs.
dcollins answer: Any item not listed in the DMG is a house rule.
Informative answer: Well, it's better than any one Feat, which should make it worth a pretty good amount. On the other hand, you have the boots of Springing and Striding (errata'd to 6k cost) that double (max +50') movement, and Boots of Skating (PsiHB) that adds 15' on level movement (30' downhill, 0' uphill) for ~2k. It'd be somewhere in that neighborhood.
Just make sure you specify that it's an Enhancement bonus to movement, so that it doesn't stack with Expeditious Retreat.