These are interesting, as they are the opposite of modern stamps, that always have a complicated design.
Here are some:
Albania 1GR 1913

Quite a simple design. These also cost a lot.
early Papal States stamps

Not so simple, but much simpler than many.
Romania first stamps 1858

(Not cheap, but quite interesting)
Portugal early stamps

Probably the most complicated desing here, but also the cheapest (25 reis ~ 15€)
Finland first stamps

These 5p and 10p stamps were quite simple too.
Bermuda

This (again) is not a cheap one. Costs more than a house.
Uganda

Probably the most simple stamp.
There are many more, for example Serbian, Brazilian, afghan, dominican stamps. Also from not-so-known places like guana, tolima, natal, new state and reunion and corboda.