Single handle valve and trim kits run from the mid $50 rage for a really cheap one to as much as you care to pay.

Around $150 for the valve and trim is not bad

If you DIY, and choose box store builder grade components, you can do this for under 1K. Upgrading any of the components or paying labor will bring the price up, often substantially. With medium grade components and paying for labor, you're probably in the 4-5K range.

Depends on where you are. Also. replacing that tub adds to the plumbing cost. How about leaving the tub, installing new fixtures and tiling? Is the tub in good shape?

Sounds like you are going to do the whole meal deal, but Delta sells a "Reno Adaptor Plate". I recall Home Depot has it for about $30. It lets you cover the existing two faucet fittings, and fit a central one faucet (Delta) fitting. No need to retile if you go that route.

I'm not sure how you could DIY this one to $1000.

As said the trim/valve for $150. There are not that many other parts. But you have no idea what is back there. Probably copper but it might not be. I'd expect that a plumber would charge $300 around here to swap it out if there was nothing unexpected. That would be with you getting exposure and covering up which would usually mean drywall behind the valve and then something to cover the holes in the tile.

We use Moen valves and faucets. A professional, licensed plumber, experienced in performing a tub/shower valve replacements can cut the tile with a grinder or multi-tool, install a new Moen valve and install a Moen repair plate to cover the holes from the previous two handle valve. If performed correctly, there will be no evidence that there was a two handle valve except for the repair plate. It is recommended that you have a licensed plumber perform this type of work, as it will require soldering in the wall in a confined area. This is not something the average DIYer should attempt. Typical cost $800-$1200. Trust me, it's worth it. We have to bail DIYer's out all the time who try projects like this and get in way over their heads.

Do not go with the cheapest tub and shower faucet. It is not an easy job to replace one, once it is installed. Purchase the best that you can afford. You only want to do this once. Ask your plumber for recommendations. Purchase it through your plumber, if possible, he will usually stand behind it if there is a problem. He will not warranty one that you provide.

If you are going to remove the tile and tub, then the valve replacement is much simpler, however, you are turning this into a major project. Pulling the tub usually results in drain issues and flooring issues. Most of the time, this requires floor repairs or replacement and drain replacements. In most areas of the country, there are also ordinances requiring this work to be performed by a licensed plumbing contractor with a permit. If you hire a contractor, please make sure they have a plumbing license and will procure all necessary permits and inspections that your local municipality requires. If they will not do this, look elsewhere, no matter how cheap they are.
Good Luck with your project.

Here is a link that might be useful: Lemay Plumbing, Inc., a St. Louis Plumbing Contractor

Ronaka--can you find a link to that Delta product?

I haven't been able to find it, and I'm really intrigued.