Several hundred dollars to replace the valve seals ONLY, head doesn't have to come off the engine. $1,500 or more to replace seals AND guides, head must be removed. If the guides are severely worn the seal replacement will bring only temporary reduction in oil consumption. At 1 qt per 700 miles, chances are that there's still no visible blue smoke from your exhaust. It's OK to keep adding oil until consumption increases further and/or spark plug fouling becomes an issue.