Actually, I think I modeled this correctly in my game a few years back - consider that, in my game Fire On The Suns (FOTS), a freighter of any decent size is going to run you on the order of a couple dozen RPs (resource points). Any decent planet is going to have an average value output of around 200 RPs per turn. A typical, well-trained and equipped military unit is going to cost another dozen or more RPs and require at least one transport freighter to carry it to the target. In order to have enough force to successfully insure a successful invasion, you need at least a dozen or more ground units (and accompanying support) in the least case (many more if you're trying to invade a home world).
Finally, the world may not just surrender even if your ground units wipe out all organized resistance (unlikely as surviving defenders which successfully disengage immediately "go bush" and become cadre forces which can organize against the invading force). You could end up in a years-long effort to garrison and control an enemy planet which drains RPs out of your overall imperial budget for years.
Many players in FOTS have just found it cheaper and easier to "nuke the site from orbit" and then move on (habitable worlds tend to be plentiful in the game depending on your species environmental requirements).
