This cost depends on a couple of things: the weight of the payload that you're flying, the manufacturer, and your desired max altitude. The recipe for max altitude is huge balloon + small payload + minimum lift gas. Latex balloons pop at a given volume (when the elastic limit of the envelope material is reached); a heavy payload needs more gas to get off the ground, so you're closer to the max volume before you let go of the payload. Also, the EDGE team keeps two balloons on hand for each flight, in case something goes wrong with the balloon on the ground.

Balloons are sold by mass. A 600g balloon will take a 2-3 pound payload to about 80,000 feet when filled with 4-5 pounds of lift (measured at the neck), though it depends on the uniformity of the balloon. Small balloons can be used with heavy payloads, they'll just pop at lower altitudes. At present, we fly the 1600g Hwoyee. It runs on the expensive end of the scale, but its altitude and burst characteristics are fantastic, routinely hitting 110,000 feet before burst with exempt payloads.

The other option that we're familiar with is Totex, distributed by Kaymont. Generally, Totex balloons are a less expensive, but they are a little less uniform and are more prone to tangling the parachute with latex debris on burst.  However, it must be noted that Kaymont has outstanding customer support, and their webpage is significantly more instructive.