If you have the belts tight, there is almost zero backlash at zero load. As you load it, the belts stretch and you get backlash. How much, not sure, that would depend on the type of belts you are using and the loads your setup will see. Probably a question for an application engineer who sells the belts. Also certain belt profiles are more prone to backlash than others, might check into that as well.
Have you considered a low backlash worm drive? They will easily give you the ratio you need and can have very low backlash. Might also check out ebay for planetary drives. About 6 months ago, I picked up a big nema 42 planetary for something like $30. Its about a $2,000 piece and in good condition. Anything with gears will give you more rigidity as long as the backlash is low.