So, I've brazed a couple racks in the past year using a BernzOmatic TS800, https://tinyurl.com/9yk32w2, with MAP gas and 45% silver/white flux. I would like to redo them or maybe make another.
The problem is, of course,the high cost of silver - I think I've been paying $54/Oz.
Can I get away with using brass or bronze filler?

These gasses will braze bronze. Particularly if you contain the heat. But for what silver costs, and the scale you are hinting at, I would pick up an ounce off the bay for far less than your quote.

I've built racks with this burner and brass filler. It kinda sucks, but it works.

I can only add that: 1) The MAPP is seemed surprisely more expensive per job, 2) Tank capacity is not much, potentially leaving one stuck with join half finished when a tank reaches empty and the resulting in angst of having to reheat after changing tanks, 3) I found it difficult to get enough heat to pull the brass all the way around a lugged joint before a tank expired. In the big picture, I think it can reduce a builders gut confidence / feel that a finished joint is 100% right. So for little jobs like racks, I think it is a ok starter. But as soon as feasible, at least a small set of tanks with a real torch should be a top priority (Amazon lists several as "Portbable Welding sets" that look similar to the small Victor set I got the first time (way back when). Might be able to get a micro-loan for the tools (Google on that). Another option might be to see if one of the local welding supply stores has a used set at a good price or if they would consider leasing/renting them. Might be a chance that one of those good folks would be willing to trade off the cost of the toolset as an offset for a nice mixte frame for his daughter/wife/sweetheart's birthday/anniversary/etc... Worth asking, but it might take bit of social engineering there. Oh one last think; I did only two frames with MAPP setup way back and it kept setting off the smoke alarm. When I had switched to proper tools, I rarely had an alarm go off. I can't prove the cause and effect 100%, but seemed to be 'obviously' so. Hope all this helps or adds.

Silver really does work with MAPP, on racks and such. When it is right it just pops in a fillet perfectly formed.

One thing to consider when using cheap aspirated burners is that it is difficult to control flame quality. You want a carborating flame that does not add oxygen to the metal. To get that you need an adjustable jet, and most of the hand helds do not have that. It also pays to have massive overkill in BTU, so you don't need to just go to an oxidizing flame to max out the BTUs. There are fairly cheap burners of this type, but they are hardly worth it, since one can normally get an Oxy set up for reasonably cheap, and then have it all.

I use a propane torch, and bottled gas. I would love to use nat gas, and a concentrator, to completely eliminate the high pressure tanks.

Another thing to consider is that the paradigm with oxy is that it is so hot, you have to keep the flame moving, and you will have the heat you need the second you hit a small part, like a rack, and you can move it around, and the flame is carborating, and moving it around you can pull filler where you want to steer it. It is hard to get all that happening, and keep the flux nice with propane.

My thought on all that with MAPP and propane is that, it does not work. A better idea might be to hit all sides at once get them to temp and then fire in the silver and quit. This means multiple heads. There are two headed torches, but probably easier is just two tocrhes. Third hand deal. This is how the factories do it with propane.