The employer pays the same social security and medicare taxes as the employee and also has a cap on the social security part, plus in some cases an unemployment tax.

If you want to include employer taxes, to calculate if the employer wants to pay you 250K how much you would get, then it's be about 42%. At 100K, it'd be about 38% since you don't reach the SS cap.

Of course, there's a couple other tricks in there, like the government can embed some tax into the healthcare costs which are required for employers to buy, but then there's also deductions that are hard to turn into a percentage like this.