Unless you're just talking about a minisplit or something $5k is remarkably cheap for a full AC install. Theres a big difference between throwing a window unit in and having an actual AC unit installed. You can't just slap a second compressor on an existing condenser. It would need a second condenser or an upgrade and second circuit run for your existing one, plus running refrigerant pipes, likely running new wiring, ventilation, controls, refeigerant costs and system tuning. A window AC unit isn't even in the same ballpark as far as price goes.