The best thing to do is go online and look at the least expensive field boots out there. Once you have a brand and model in mind you can price shop using that as your search. Some places to start: Big Dees, Schneider's, Chick, Valley Vet Supply, Dover Saddlery, State Line Tack.
There are only so many companies that make hunt/field boots and most of these places carry the same brands.
Be careful to read the description. Some are synthetic and are less expensive but they aren't always a good choice.
Synthetic boots look nice and keep their shine. They are also waterproof but
they do not
break in at the ankle. This makes them quite uncomfortable to ride in and your daughter will have a lot of trouble keeping her heels down.
When you buy real leather, she will be able to clean them and oil them. I like Neatsfoot NF as it isn't as greasy as some. The oil pushes the moisture out of the leather and keeps it supple. When she outgrows them you probably can find another parent at the stable who will buy them from you.
Good luck and have fun shopping!