Your $20,000 quote may be reasonable assuming you're using Owens Corning Duration shingles. I've contracted to have my roof replaced for $25,300 after 21 years using a Certainteed master installer. Roof is about 50 squares with medium complexity for a 1.5 story home, 3,460 square feet. I opted for a "Fortified" roof due to the waterfront location in NC, using Certainteed Landmark Pro architectural shingles, Certainteed synthetic underlayment, stainless steel roofing nails and other upgrades in materials and installation techniques. My logic: I've already had one $1,200 repair near a valley where the plywood rotted and now I have another suspect area just like it. Original owners, we may keep the home for 5-6 years. I'd rather shell out money for a new roof now and enjoy the security and peace of mind for 5 years compared to waiting until selling time and having potential buyers ding me for replacing the roof for them. Like the Fram commercial said, pay me now or pay me later. Prices later will only be higher. I got three other quotes ranging from $24,000 to $30,000. Good luck with your project.