For the hours you mentioned Changi Airport is fine even without a lounge, I did that recently with an even later connecting flight.

Is $50 worth it? It depends on what you need the lounge for. Do you want just a quiet place to chill, or are you looking mainly at food and drinks? A food court with many options at good prices will deal with the former, but if you are after some alcoholic drinks then those can be quite expensive at the airport. Then again there is a 7-11 airside and if you decide to enter the country then there is a decent Fairprice store at the basement of T3.