On the lower end for pricing look at Riedell Dart or R3 skates, like these: https://www.amazon.com/Riedell-Skates-Roller-Indoor-Outdoor/dp/B00655BHXS. They are more expensive than the ones you linked, but more appropriate to the task. Look around, you might find them cheaper, or you may find someone in a local league who wants to sell their old pair.

If you really get invested in derby, you can spend a lot more money on skates at another time...