yes there are forged and cast rims, the cheap ones are cast and the majority come from china or sumthin like that, the good stuff is forged that comes from japan. manufacturing proccess will determine price as well as supply and demand and brand.
the benefits of forged rims would be weight and strenght. if your going to run a low pro tire forged or not you hit a big pot hole the rim will bend, how much it would bent would be the difference between forged and cast. also forged rims are less likely to crack in these cases becasue of the stronger bonds in the metal.
the upper end "race" wheels are most always forged, usually 1 piece but there are 2 and 3 piece wheels that are forged too.
now to the cast rims, gravity cast rims are the cheapest and most crude way of manufacturing a rim (those are the $100 rims) there are many other ways of casting rims, like vacum casting which is better, i dont even know half of them, there are also rims that are cold worked in to shape which is better than cast but not as good as forged. cold worked meaning shaped in to a rim while the metal is cold kind of like stamping steel. the benefits of this is that the metal used is usually good close to forged quality, these are probbably the inbetween priced rims.
well thats all i can spit up hope this helps