It's all gonna count on size, color, and who you buy from.
If you buy from some average Joe in the paper, you could get a nice little jack for $400 probably, most likely will mature 33-34" minimum, gray, light brown or maybe a spot if you're lucky. A jenny for $600 for a foal, maybe $700 for a bred one, same sizes and colors.
NOW, if you go to an actual mini donkey breeder, you'll pay the same prices as the good show quality miniature horses. Blacks are hot right now, especially small ones. The high selling jennet at the Ohio Select sale in 05 sold for $25,000, her half sister for $10,000. But that is a bit extreme! 32-33" sorrels, blacks, spots and dark browns with very very nice pedigrees will START at $1500 for jack foals, $2000 for jennet foals, bred jennets usually $2500. Under 31" in those colors, especially under 30", go for a lot more. Foals usually START at $3000. We paid $1500 for our sorrel jack as a suckling 2 years ago, from an unknown first time breeder (altho they bought their parents from a big farm, hence the price). Good pedigree and conformation, average sized (33" now).
Our jennets that have a pedigree behind them start at $1000, and prices go up all depending on pedigree, conformation, size and color. A jack prospect would start about the same, geldings are considerably less at $500.
Our first jennet we bought, 6 years ago, we bought in a package deal with her jack mate for $400. This jennet has paid for herself many, many, many times over in her 3 foals. The jack we sold to a friend for $200, so technically, she cost us $200 and a tank of gas to go get her. Sometimes ya just get real lucky!