One of the cheap options the "Doodle creatures". Very simple linework, but still can be really colorful, and with added stuff can be look really cool. But without it still looks good~
I didn't tried out that many creatures in this style and I have a collection of my own weird and silly "doodle creatures" what I created in school. Yes my books full of these things... ^^"
If you like simple art stuff, you can use this option.

Sample art!!!