In this aricle, there is a comparison between a few cameras under $500 for serious photographers. There are many reasons to buy a compact camera and you can read about them in my article Canon G7x review.
What you should look for
If you are looking for a compact camera you should first think what the primary uses will be. Should it be very small? Does it need a flip screen for vlogging? This list of features you may want to consider are also in the article about the Canon G7x and contains the most important things you should look for prior to buying a compact camera. I also recommend to going to a camera store to play around with some of the models to see which you like the most.
|RAW||The camera should make RAW images. It is, as I mentioned earlier the most important thing for an advanced photographer.|
|Small and Light||Buy something small and light. You already have a big heavy DSLR and you want something that you can always have with you.|
|Control Settings||Check if you can control the camera like a DSLR (ISO, aperture, shutter speed).|
|Additional Gear||If you want to make special things like underwater photography, check if the gear is available before you buy the camera.|
|Same Brand as you main Camera||When you have a big camera you should look at a compact camera that from the same brand. The user experience is similar to the bigger cameras.|
|Big Aperture||You are losing light because the sensor is smaller. So be sure that you choose a camera with a big aperture.|
There are some features that you may want to have in the camera that are a bit special. These could be a viewfinder, a hot shoe, or connectivity with Bluetooth or W-Lan. 4k Video is also a feature that you need these days if you are vlogging or selling stock videos.
Compact cameras under $500
Panasonic Lumix DC-ZS70K
The Panasonic Lumix DC-ZS70K is a really great compact camera. It has a 20MP sensor and a 3 inch flip screen. It can make RAW images and also 4k videos. You can get in on Amazon for under 450$. This may be the best option for a professional compact camera under $500. If you are from Europe you can find it as the Panasonic Lumix DMC-TZ101.
Canon G9x Mark II
An other option but a bit less capable is the Canon G9x Mark II, the little brother of the Canon G7x. It has a really capable sensor and is a b bit smaller. It comes with a 20MP sensor, does RAW and is available on Amazon for $450. Unfortunately it has no flip screen an does not do 4k video.
There are also some other cameras which have more features but cannot be found under $500 new but if you buy a used one these are also great options. Sometimes an older version of the camera can also be a good option. Most time the manufacturers do not change many things. They put a faster processor in it and give it another name but the image quality is comparable to the older versions.
Used Sony RX 100
The last option is a used Sony RX 100. Before you buy check what version you have selected. Sony is changing only one number in the model name. The newest one is the RX 100 V witch is nearly $1000 but you can find a used RX 100 IV for about $500. I do not recommend buying an older version than the RX 100 IV. The RX 100 is one of the best compact cameras you can find in the market. It has a great image quality (20 MP Sensor) along with a 3 inch flip screen.
There are a lot of other compact cameras on the market, but most of them are more expansive or are not capable enough for serious photographers. From other Brands like Nikon, Olympus or Fuji I did not find an actual model that would fit in this class of cameras.