The difference in cost will reflect the amount of work done and the ability to rectify faults during the service. If I stamp the logbook then the service is done as per the logbook. If you sell the car the new owner will expect that the servicing has been done properly.
Sure there are numerous workshops or mobile guys who can 'service' the car, possibly can do it for uunder $200 if you push them. When you ask if the fuel filter has been changed they will look blankly at you. When you ask how the cabin filter looked they will ask you what the fuck a cabin filter is. A lot of mobile guys will not rotate the tyres because they are not equippped to do so. Some suck the oil out the dipstick tube instead of draining the sump plug. Did they use Subaru upper engine cleaner? There are numerous Subaru traits that you need to know, adjusting the handbrake isn't as simple as it sounds, bleeding the cooling system can cost you an engine if not done properly (almost impossible without the right tools) The CV boots, sump plug washers etc are all specific, cheap generic ones just don't work properly.
If your budget is tight I can slim down the service, clean the air and cabin filters instead of replacing them, skip the fuel filter, skip the upper engine cleaner etc. and jst make a note in the book so you can catch up next time.