RE: When is it time to change a Tx battery ?
I would never buy a cell phone that had a lipoly in it.
Fortunately most of them have a LIFE battery and that is a different animal. You should consider one of those for your transmitter. LIFE batteries don't have the low internal resistance that a lipoy does but you don't need that in a transmitter.
The prices you quoted for FMA chargers were for refurbished units. I don't think they are still available. I had to pay $189 for my 10s when they were new. They have a new 10s available now. I bet you are not going to get that for $100.