Standard Gauge tinplate is going to be more costly, on average, than O - just because of the size if nothing else. But Pappy is right, there are some very good deals to be had in MTH reproduction sets.
I range from vintage Ives in Standard Gauge, where you are looking at sometimes $1000 for a passenger car, through the modern reproductions, to prewar Marx O gauge tinplate which can be had for $10 a car. When I started I didn't even know about McCoy, but they have gradually become one of my favorites. They were mostly made in the 1970's or thereabouts. The style is like the very early Lionel 10 series freights: quite toylike and also quite large in scale.
Most of the McCoy designs are really unique, and sometimes whimsical. They have some very nice trolleys and interurbans, a very cool handcar, steam and electric locomotives, freight and passenger sets. Some of their locomotives, like the Chief Cle Elum steam or the big E2 electric, are at least as good as, and maybe better, than anything else out there. And you are right about the prices, it is possible, with patience, to find McCoy freight cars for $40.
There is a limited number of McCoy trains, and it is a small niche with not too many serious collectors, as compared to the bigger manufacturers. Bob McCoy Jr. and his wife Bonnie are still selling inventory, and have parts and will do repairs. They do some selling on ebay as "mccoyswoman".
There were problems with some of the castings in McCoy motors, so always check to see if these run or have been replaced. Everything I have bought from Bob and Bonnie has been repowered with newer motors that they have put in. Others I have found that had the motor casting problem, I sent to them and they put a new motor in at a reasonable price.
Other than that, I find all the McCoy products to be very well made with high quality finish and graphics. They use the universal hook coupler, they are built of heavy tin stock that seems to resist bending, and they are enameled with silkscreened lettering and graphics. They have big trucks and wheels like the Lionel 10 series freight.
McCoy originally went in business making reproductions of early Lionel, but people got upset with him because it was extremely difficult to tell his repros from the originals - they were that good, and I think that speaks to McCoy quality. As a result, in the late 1960's he started making his own designs, which could not be confused with older trains.
Much of the McCoy line consists of cars made for TCA and TTOS conventions and other events. This is what you most often find on ebay, and they bring the lowest prices. Personally, I collect the McCoy trains that are not convention or division graphics - McCoy trains carry a number of heralds, some authentic (Great Northern), some whimsical, and many with themes from the Pacific Northwest area (McCoy is based near Seattle). It is a great line for collectors because McCoy often made only a few (sometimes only a hundred or two) of many variations, so there is always something new to discover.
McCoy standard gauge was a wonderful discovery for me and it could be for you too if you like the style.