Well its casual it should be if she enjoys it.
If you want a cheap cost removal spell there is
walk the plank at the time of this post I think they are going for 15-25 cents non foil. To play they cost 2 black mana at instant. Granted they cant kill merfolk but that is a very exact creature card. Most who play that would be in edh/standard/or modern.
One of my favorite scanvage cards is
slitherhead I think are going for 10-15 cents. They cost either 1 green or black. And when they are in the graveyard you can pay 0 and
exile it to put a +1/+1 counter on a creature. It may not win games but can both be a chump blocker and a support.
A big problem is you have many ways to put things in the graveyard but no retreaval.
Rise from the grave could help with that it the price range of 10-50 cents. Not the best option but a low price cost one.
If you want something that can weaken fliers or an opponent in general there is always
obelisk spider. 15-30 cents
Fog is a life saver at instant speed 1 green it prevents all combat damage that turn. 10-45 cents
If you want to troll a player there is perminating mass. For 1 green you get a 1/3 who any creature that battles it becomes perminaing mass. With a 10-20 cent price range.
Going back to removal there is
acidic slime. Priced around 15-25 cents. For a 2 green 3 any color you get a 2/2 deathtouch when he enters the battle field destroy an artifact, enchantment or land.
For a
simple search card there is one of my standard favorites
Adventurous Impulse. For 1 green you look at the top 3 cards of your deck select a creature or land and put it in your hand then put the rest on the bottom of your deck in any order. With a price range 08-16 cents
One last card of mention is gravewalker. For 20-50 cents you get a 5/5 flyer who can bring back stuff from the graveyard each turn for an additional cost.
I'm not telling you how to make a deck or how you like to play. But here are some cheap cards I think you might like or consider for the deck :3