It keeps getting more expensive to buy at auction. Christie’s has quietly raised the fee it charges buyers — for the second time in a year. This increase targets low-end purchases.Beginning on Sept. 1 buyers will be charged 25 percent, increased from 20 percent, on objects costing up to $20,000. Charges on prices above $20,000 remain unchanged: 20 percent on the sale amount from $20,000 to $500,000, and 12 percent on any amount above that.