Others may have a more detailed take, but I think of a tandem like buying two single bikes. So for your $750-$1000 single bike, you’re at a $1500-$2000 tandem. If you wonder why this is so, look at the prices of the tandem-specific parts you need to build one up. Crankset F&R, eccentric BB, 145mm rear hub, probably 40* rims, stoker stem, etc. All costs money and adds up.