Ok, so I goofed around a bit with High Guard, and ended up with two different stretched 300 dt yacht designs.
For both of them I wanted to keep as much of the original vessel design as possible, increasing the hull with 100 dtons and replacing the drive and power units as well as installing a larger bridge (required when ship size exceeds 200 dt).
I haven't looked at the economics of either version, but to be honest, if you need to ask "how much?" You probably can't afford it anyways
First off, for the speedsters, we have a jump-2 capable yacht with insanely impressive thrust 6 manoeuver drives - and it's even got room for 10 more dtons of cargo/vehicles/fun stuff!
What? Having the fastest ship in the spaceport isn't enough? Your pocket empire is so large you're forced to spend way too much time in transit? Or perhaps you just wanna travel, see the galaxy? Well look no further, we've managed to squeeze a 3 parsec jump drive into the extended yacht hull, while keeping the luxurious interior the base model is known and loved for! The one thing you do miss out on is the air/raft garage. And speed. The jump-3 yacht retains the original type Y:s 1 G drives. It does still come with a boat though, for when time is of the essence in-system.
The "speed yacht" has jump-2, thrust-6, 20 dt power plant and 62 dt fuel (j-2 + 4 weeks). This together with the larger bridge adds up to 90 dt, leaving 10 dt for extra cargo.
The long range yacht, with jump-3, thrust-1,92 dt fuel has a smaller power plant than the speedster, but still adds up to 105 additional dt compared to the regular type Y, and thus the air/raft had to go (or the cargo bay could become smaller, if that's preferred).
Of course there are other possible designs, a less powerful m-drive on the jump-2 version means a smaller power plant and more space for luxury or cargo. Bringing advanced tech into the equation can make components smaller, and if the hull is increased to 400 dt rather than 300 there should be space for both better m and j drives, and so on...