For modular cyberlimbs, the essence cost multiplier is listed as 1 and the cost multiplier is listed as 1.1. I am assuming that this means the essence remains unchanged and the cost increases by 10%, but I could also read it as an additional cost of x essence and 1.1 times the original value and then added on. Am I being a complete idiot in my confusion?

Yeah, you're reading too far into that. Make sure not to confuse the terms Cost and Essence Cost.