Common Towers are a very good weapon in soulkeep and it is not about being free to play with but they could do a good work when compared to rare and epic, it is closely to their action and in this case you could see that they are amazing cards to be with you and if you want to rent I suggest to get an epic cards from the market but for spells.
Epic and Rare spells in the market are worth a try to rent them and to get them in a good stats as common towers could be played with and give the same effect as rare and epic towers and this is in the physical and magical damage but features of Slime Trooper for Example are different.
As You know that common spells give some features and Rare and Epic ones gives a different features like Mana Mania that could increase the mana earned from the battle and when eliminating fiends and also cheap is like its effect but different from it.
You could also find that there are spells like higher ground and it is a rare one which is not find in common spells and its effect is not like it as it could increase the range of tower so if you want a spells to try something that is completely different then you could check the market for spells not for towers.
