It sounds very fair to me. You have a four year old mower. I don't know what the warranty is on your mower but if it is pass time it sounds to me like the dealer is helping you out. Its the manufacture that pays the dealer for warranty work and not the dealers fault if the manufacture only pays for the part and not labor. I have worked for dealers in the pass and if it was out of warranty the owner paid the bill, end of story. The mechanic doing the job needs his pay check just like everybody else that works for a living.