Ummf! ...affordable being a relative term...D.O meters are a quite a 'high price' item...
I'm sure they combine them in one probe together, but it will cost you more.
If you have a problem with D.O. in your aquarium or pond consider adding a larger air bubbler. You can also get simple hand field test kits. To test the DO. that are easy to use. I'm not sure the brand name or where to get them I would have to look it up, but I know they make them because I have used them before for IOWA WATER.
LOL.... I have had this icon for a long time. (It's a dead fish with a rose, for all of the fish I lost in my years of keeping fish.) I just joined this website to contact a member about buying talipia fish and of course help other people out!
I can't justify the cost. Halemart shopped around a year or more ago - you might want to look back at his posts to see what he bought. I remember that he posted about the meter he purchased. He'll probably surface and respond.
I experimented recently with aeration, using a DC pump and at one point my fish abruptly stopped feeding. Water tests of ammonia/nitrite were good so I concluded not enough DO. I went back to a different method of aeration and feeding returned to normal. Adequate aeration is a wonderful thing and bad things can happen quickly if you don't have it.
My thinking is that you need two sources of aeration, on two different sources of power, just in case one source fails.
If I got off topic in any way, too bad and good luck.
99.99% of good DO meters... record temperature as well...
cheap ones go from about $300... reliable aquaculture meters... $600-1200....