My question is actually how much MORE it should cost per sq ft to install on diagonal vs installing it the straight way...

Your builder already told you what it will cost you ... $1 to $2 per square foot.

The original cost of the tile has nothing to do with it.

Diagonal requires meticulous cuts with the tile saw, usually has more waste, and is slower to lay out and to lay. More work = more money

Thank you for your insight lazygardens. I know it should cost more because it IS more work & materials. I was asking if that amount seems in line with the average. That's what these forums are for-people who may not know about homebuilding and need input from others. it's not an unreasonable question... I'm wondering if I should approach him and say 'hmmm...that seems a bit high." Or just make a decision based on the quote he gave me.

" it's not an unreasonable question."

??
No one said it was!

Talk to your friends/neighbors to see what the going rate is in your area. What may be the norm in my area could be totally different than in yours.
Since you think that guy is high, have another one bid on the job.

You will also need more tile because there's more wastage. So make sure you order at least 15% extra rather than the standard 10%. For large format tile like the 18x18, if it's a smaller space with lots of cuts, you may even want to order 20-25% more. It's a lot more labor intensive, and for a 12x12, $1 a square extra is cheap, and so is the $2 a foot for the 18x18. You just doubled the time that a small bathroom will take to do, but only went up $2 on the labor. That's a bargain.

The big issue with diagonal cuts is that cutting one tile does not produce two useable equal halves.

The wet saw makes a kerf .

If the room size works out well, the triangles at the edges are all less than half a tile so you CAN get two out of each piece.

Thank you for your responses. Sounds like that is the going rate. It seemed like a lot to me so I'm glad I asked. I think I'm going to nix the diagonal tile and maybe use the $ towards the lighting in the rooms, something I think might make more of a statement than the direction of the tile. I appreciate the input.

Tile installation can vary by as much as $4/s.f. depending on the difficulty, type of tile, setting bed and the market.

How will the final cost be determined, by the actual time? If so you need to know that unit price.

I would expect a 30 to 50% markup for a diagonal pattern because it will probably take that much longer to do.

Lay the pattern out carefully to avoid goofy little pieces at the edges. This approach can easily backfire in a small space. I recommend a smaller tile in order to create a stronger more uniform design that "ties the room together", like a 1x2 basket weave.

Thanks Renovator8- you have such good insight and I always appreciate your advice. :)

I guess I've been getting a deal for years!

My tilesetter doesn't charge more for laying tiles on the diagonal than the boring patterns most everyone else uses. Only for multi-tile patterns--where a number of different size tiles are laid in a repeating pattern--which, he finds, requires a second tilesetter to keep an eye on what's going down.

If you nix diagonal on the floors, keep it in mind for backsplashes.