$100-$160. Depends on how accessible it is, and whether it’s a 2 piece or 1 piece.

I'd be surprised if you could get a competent plumber to do that job cheaply in a major metropolitan area. That's why I do it myself: it costs about $2 for a new wax ring, and takes me about 45-60 minutes. There are plenty of videos on YouTube that show how easy it is. If you don't want to do this yourself, the install could cost as much as or more than the toilet itself.

We paid about $300 each to remove and dispose of 3 toilets, install new 2-piece toilets. The toilets themselves cost about $300 each.

It's easy to replace a toilet IF you have the strength for it -- toilets are heavy! And you not only have to get the new one in the house and installed, you have to drag out the old one. DH has done it multiple times, but he's now getting to the point where it's too much for him.

Last time I paid a plumber was quite a few years ago, and it was at least a couple hundred bucks in a metro area.

If you are able to DIY (I would not be), then a 2-piece toilet will be easier to handle weight-wise, especially if this is for an upstairs bathroom.

For the cost of a plumber to do the installation - depends on your locale and its COL. The above-mentioned quote of $100-160 in my area would just cover the plumber walking in the door. The cost in my area is about $250-300 each.

cannot DIY and want to use a plumber because this is an older home and need to make sure plumbing is ok. Have been quoted $500 to install plus provide new toilet but that seems high

Yes, seems a bit high, but a lot of things are more expensive during covid times. If you can't DIY, get more than one estimate, or get a knowledgeable friend to do it. It's not rocket science....

Depends on the toilet also. Our plumber said that he charges an extra $200 for a Kohler 1 piece because they are so heavy to bring in.

It is easy to get a quote from a plumber for installing a toilet especially if you have selected the toilet model so they know the logistics.

The only way to really find out what the going rate in your location is would be to call several reputable plumbing companies.

In my area, handyman charge is around $150 and often includes picking up the toilet at the store and taking old toilet outside for disposal. 
This might be a good opportunity to test out a plumber before an emergency happens. Ask around for recommendations and call a couple small shops to check out price and service. I don't want to call anyone who advertises widely.  I'm looking for the guy who leaves business cards with previous customers who happily pass them on. I don't like big companies because I want to hear the same voice and see the same face next time and I want to be a known destination to them also, not just another phone message to return. Plus, if you are a repeat customer, you are more likely to get a visit in case of widespread emergency demand.

Depands where you are. Here a plumber won't answer the phone for $100. I'd expect the charges to end up in the $3-500 range. Getting a couple estimates is the only way to find out. If it turns out you have a broken flange or similar (not uncommon in an old house and you won't know until you remove the old toilet) you need to be ready to add another hour's work.