Access and MySQL are free, thats why companies are using them, but there are several things you need to consider, MySQL doesnt support VIEWS or STORED PROCEDURES, and SQL Server or Oracle do, and believe me a SP can be very helpful processing data or a view, helping you to avoid repeating the same data, or create complex queries in the client side.
An engine, and many more things can make you think what DB you should use, here in the company weve 90 SQL Server servers and 1 Oracle server on a True Unix server (Alpha server) only for California (for the whole country there are many many many more servers in the datacenter in Houston), and this will be amazing, but Oracle is the slowest server.
So all depends on what you really need, and hoping that the company will be willing to spend a few thousands in a good DB, and how good DB designers you have, you can have the best DB in the world and a mediocre DB designer, the result will be like the Oracle server here (thats why Im removing this server soon (I hope))