Question about compression and needed repairs

My inline six cylinder had a check engine light on. It was diagnosed and found to have half of a cup of oil in the spark plug cavity. When the oil was cleaned out and parts degreased, the misfiring while idling stopped. However a compression test revealed 40% compression or was it/40% less compression on one cylinder number one. Mechanic thinks that the head gasket should be replaced and believes that the cylinder head should be checked for the correct fit/clearance first. Since a back yard mechanic did valve train work last year and he failed to have the parts sent to a machinist to check for trueness, he says that if four to five hours of adjusting the cylinder head fails to remedy the compression issue it may possibly need an expensive valve job. Does this sound reasonable and if so, what should I expect to pay for the first job? He says that it may only need one valve addressed but if more, it will cost more, hence the $2,300.00 price tag for twenty three hours or more of labor. This car is probably not work more that $5,000.00 on the used market. Thanks in advance for your advice, and no, I cannot afford lease payments on a new car. :) I am a single mom trying to make a wise choice.