So probably the closest comparison is the carpet of flying. For 20k, I can get a carpet with a fly speed of 50ft (25 ft base, and then if its just me its pretty reasonable to get the double speed boost, unless I'm massive or carrying a crazy amount of gear). You could also look at the 15k carpet for 30 ft speed, but less likely to get the double speed to 60.
This is completely at-will fly, does not require an action past the initial activation. I can stand on the carpet, fight on the carpet, haul loads with the carpet, etc
The 120 ft "speed" of the grappling hook is nice, but it consumes your entire turn (action + bonus action to real + reaction to get the hook back), it requires the destination not be blocked by total cover (unlike the carpet that could fly around as long as there is some opening), and the hook requires elevated surfaces, a carpet can go as high as I want. Oh and there is a miss chance, even a 13 AC is not an automatic success with an attack roll, even a high level character might have a 10-15% chance of failure.
So compared to a 20k carpet of flying, the 25k grappling hook is a very very poor deal in my opinion.