Laura, I replaced the axle under a Burro
17' a couple of years ago. I replaced the stock Dexter #9 with a Dexter #10 3,500 lb axle. The axle with 10" brakes
cost me about USD $250 delivered to Midwest Wheel in Cedar Rapids, Ia and I had a local welder to the cutoff and install for an additional USD $200 in labor. Mine necessitated new wheels and tires
as well as I changed from a 4 lug to 5 lug pattern so I had a little extra cost there.
Axle replacement costs vary greatly, and vary with who orders the axle, what brand they get, and who does the welding. Ordering it yourself and finding a competent welder to do the job is probably the least expensive way to go. Taking it to a dealer is probably the most expensive.