super high end ones that i don't know anything about:
10000, 11000, 14000, etc. these are all $1200+ new, used they tend to pop up anywhere from $200 to $1000+. these are the clinical grade models used for professional rehab. the suck part is that the accessories cost like 8-10x as much as they do for the consumer grade models, so unless it comes loaded with accessories you probably won't want to buy them.
high end models (in descending order):
XLS, XL, 3000, 2000. sturdy tower, easy to adjust incline, tons of accessories, squat plate capable, high build quality. these 4 models all use the same accessories.
mid-end models (in descending order):
1800 club, 1700 club, supra. narrow tower, lower build quality, tons of accessories, squat plate capable. the supra uses the same accessories as the XL/XLS/2000/3000. the 1700/1800 can use the same accessories.
low-end models (in no particular order):
1000, 1100, 1500, achiever, elite, ultra, pro, performer, etc. narrow tower, lower build quality, minimal accessories.
XLS. new $1300, used $350-900. super wide glide board, new came with squat plate, pilates bar/kit, leg pulley, wing attachment.
XL. new $1200, used $200-400. super wide glide board, new came with squat plate, leg pulley, wing attachment. there was an informercial promo that included the pilates bar/kit or press up bars. the XL and XLS are nearly identical (the handles are the only real difference), so they are "worth" roughly the same amount imo.
3000. new $900, used $100-300. new came with squat plate, wing attachment. there was an informercial promo that included the leg pulley and press up bars. total gym logo on board is white/silver outlined in blue with no model number.
1800 club. new $350, used $200-300. new came with pilates bar, leg pulley, wing attachment, press up bars, dip bars, ab crunch boards, floor mat.
1700 club. new $350, used $150-250. new came with pilates bar, leg pulley, wing attachment, press up bars, dip bars, weight bar, floor mat.
2000. new $600, used $100-250. new came with no accessories. there was an informercial promo that included the press up bars, wing attachment, and squat plate. the handles on this model can't be changed.
the accessories for the XL/XLS/2000/3000 usually cost $20-35 each. the 1700/1800 came with pretty much everything and i don't think you can get the accessories separately without calling the manufacturer.
i would avoid the lower end models unless budget is your primary concern. in that case, you should be able to get one of them for under $60. all the standard exercises can be performed with any model. the 1500 is a solid one and probably the most common one floating around used. most low end models came with the wing attachment and leg pulley.