^^ Someone who works a the same place I do has one of the Magna Carts, and I know someone else with one that regularly hauls 20+ lbs on his, they said they paid about $20 for them but saw the prices jump to $30 last time they looked, so it seems to be a price hike trend.
About those Magna carts [not sure about the other brands], they seem pretty solid with a decent aluminum build, but if you put something heavy on them the collapsing handles feel a bit wobbly/ loose [its noticeable, but I doubt it would be a problem ever]. They are basically little hand carts and you would have to stack your stuff vertically. The other problem is they fold flat to the general shape of a thick pizza box so storing them on a bike could be a problem.
You should also consider a old and cheap folding stroller from a store like Goodwill [don't know where you could find a place like that where you are] but for $5-20 you could get a ~5lb folding stroller that would be able to take ~50-100lbs, just fold it up and strap it on the top rear rack with 2-3 cinch straps
Something like this but smaller maybe [its a $16 kmart 'Folding ''Umbrella'' Stroller']:
If you want to have panniers with wheels and you cannot find any then you could potentially modify the panniers you have. For each pannier get: 2 straight rolling castors [not the swivel kind, get solid looking metal and plastic 25lb+ load bearing ones], aluminum bar stock [probably 1"] then a box of pop-rivits and a pop-rivit tool, you will also need/ want some saw to cut and a drill to make holes in the aluminum stock and something to bend it easily (if you find someone with the tools, or convince a local store to let you build it there that would be a great option, or maybe a 'buy it, use it, and resell it to them' deal. I can post a quick sketch later, but a quick description of what it would be like would be you take about 1ft of aluminum stock, bend it ~120 degrees in the middle, drill holes for the castors at one end and pop-rivit it on there, then on the other end you would attach it to the rigid back of the panniers with pop-rivits [you may have to back it on the inside with washers or a thin sheet metal plate], when its done the aluminum stock should not interfere with the rack, and the wheels will be away from the rack/ bike wheels but be useful when you go to move the panniers around -you could strap/bungiee several of the rolling panniers together and have it act as a single cart, handling/ turning will be terrible though