How much would you price boots of Expeditious Retreat?

[ChefOrc]

Well if we assume speed bonuses have a linear progression, then my original 3k for +10 would be 9k for +30, still greater than the formulaic 4k but less than the 12k boots of speed.

Like Patryn of Elvenshae said, you have to take into account that there are two effects in the boots of springing and striding, which multiplies the cost of the secondary effect by 1.5. Depending on which effect you choose as secondary, the price for a 10' bonus would be 2000gp or 1750gp. By your formula, boots of expeditious retreat would be worth 6000gp or 5250gp.