As far as the original question goes in the modern realm leaving out AC style injector razors you don't have many options in the $100 or less realm.
For Gem razors you can buy assuming they even still make it you have the PAA Starling V2 and that is it. The other option is to go the DIY route and get a 3D printed one either doing your own printing or have someone make it for you using some freely available designs.
For injector razors you have 2 options besides DIY and that is the Supply V2 alloy version at $75 with 3 different aggression level plates or the Type L injector clones of the old Schick Type L razor that you can get on ebay, amazon, ebarbershop. Same razor different name depending the vendor. The prices on the injector clones will vary depending the vendor and how many blades are included with the razor.
For newer and discontinued but can still be had used the Mongoose Minigoose might drop below $100 in used realm. That razor takes Schick injector blades. You also have the Type O clones which are knock offs of the Schick Type O injector that still show up for sale used in the under $100 price range. You can also add the PAA Starling V1 and V2 razors here if it is discontinued.
There are some other options above $100 but those razors like the Blackland Sabre I don't think you'll ever catch them at $100 or less on sale or used.