2005 Pontiac Montana -Slipping between 2nd and 3rd gear

When accelerating between 2nd and 3rd gear, the transmitting is slipping as confirmed with a local shop. The van jerks forward and makes a "clunk" noise. The shop said my transmission filter could be dirty but the lines were hemorrhaging. The transmission also showed signs of wear. The fluids have only been changed once in our care. We received it used. I was told that without doing the recommended work, the transmission would last maybe months to a year. They quoted me under $800 to do those 2 bits of work. Does this sound correct by what I have described? There are other issue but I will do a new question. The van is work only $3500 and overall the repairs will be over $3000.