Cheap? Hehe, this is vex…
Also, what do you mean by cheap? Like do you already have any gears, or are you a new team looking for the best gears to buy?
600 rpm (blue) motors are almost always the best. There are a lot of gear ratios to choose from, as well as having smaller/shorter/more compact gearings. It also has slightly less slop and friction than gearing up 200 or 100 rpm motors.
But, there is a very nice resource that already exists if you search the forum: