Why would 2 year old battery stop holding a charge all of the sudden

Thank you. I was hoping this wasn't the answer but thought it might be. It has been some time since I have had to replace and alternator. What is the average cost of replacement on a 4wd 6 cyl Jeep liberty 2006 ltd? Is is cheaper if I purchase the part because obviously I cannot do the labor....and lastly - this was a leases auto when purchased. 50,000 miles - 1 owner had it two years I believe - #2 about 9 months in - the TPM system stopped working which you find out the hard way of course. Being a disabled vet I have to watch mu finances...and the TPM is really expensive to fix. Since this battery situation - the car is also wavering on the miles per gallon\. yesterday it was 18.2 (close to that) dropped to 5.1 after I had it jumped this morning - then dowm to 4.4 nad up to 5.6 - this along with the gas miles left fluxation is a pain in the ass. Why would that be happening? I really really appreciate your help. My heart condition is at the point where I will have to decide if I want transplant. Stress makes things worse and being able to get this from you - I cant tell you how much of a load off my mind just to have the optional answers or confirm what I thought. Not matter how much it sucks I have to know...so if you could get back to me as to why the mileage per gallon digital readout is doing the above that will answer everything that is going on. Is it at all possible that when the repairs I mentioned in the last email were being done, the could have done something by accident that could cause the alternator's or digital readout behaviour. Thanks again! Polli