Geography and density matter. It's way easier, and cheaper, to build an at-grade rail line in a low-density metro like SLC than it is to build a rail line of any type in a comparatively high-density/geography-restricted metro like Seattle.
SLC has plenty of available space in which to build it's rail line, Seattle does not. Even before construction costs become a consideration, Seattle's rail line will be more expensive because of the additional cost of acquiring the land (or easements) on which the rail line will be built.
Aquiring land to build for transit projects isn't typically an issue because these projects are either built below ground at a level that doesn't interfere with property rights, or on existing public rights of way. It can matter for stations (and that definitely gets worse the bigger and more extravagant that you build), but not the lines themselves.
I think you need to learn more about property rights and the availability of public rights of way.
Property rights extend below ground, and it is generally necessary to either acquire the land, or an easement to the underlying "subsurface". For example, in NYC, the transit authority needs to acquire "subsurface transit" easements to build subway lines, though in most cases today most deeds are already burdened by such easements as a result of historical eminent domain actions by the transit authority.
Public rights of way that are adequate for a rail line, for the most part, are historical remnants of earlier railroads and rail lines. A well-developed city without those historical remnants, like Seattle, generally doesn't have existing rights of way *on which to build a rail line." A relatively undeveloped city like SLC, with plenty of open space, doesn't have to restrict itself to using public rights of way.
Many cities have depth limits on property rights by statute. Others can and do use eminent domain to acquire a subsurface easement, and unless the property owner can prove some sort of marketable value to the undeveloped land at subway depth, eminent domain is extremely cheap. Yes, even in dense cities.
Surface light rail rights of way don't need railroads or even former rail lines in order to avoid land aqcuisition. They can use streets. This is just as valid along SLC's 200 West in downtown as it could be for Seattle's 15th Ave W, Leary Ave, Eastlake Ave, or 45th Ave N. It is completely common for new light rail systems to use a mix of both former railroad rights of way as well as surface roads.
SLC has plenty of available space in which to build it's rail line, Seattle does not. Even before construction costs become a consideration, Seattle's rail line will be more expensive because of the additional cost of acquiring the land (or easements) on which the rail line will be built.