I have 3 bids for trane, which I will probably take, but these range from $3700 to $7000. These are both 13 seer, 5 ton units, including all warranties, pan, all the same. Why the huge difference?

You need to ask the people that gave you the prices

Yes, I would ask for a bid breakdown, maybe installation costs?

I had a Trane furnace and aircon fitted a couple of years back. I gave four supplies the same equipment details, the total price ranged from $5800 to $7500. However be careful, I have heard of heating engineers agreeing to fit a certain furnace then fitting a different one without telling the home owner. When mine was fitted, I asked many times for a breakdown on parts and labour, I never got it.

Here is what you do: Get those 4 bids for Trane. Go over them real closely. Inspect and scrutinize every item on that bid sheet. Once you do that take all 4 bids and throw them in freaking garbage.

There are better options on the market that will save you some decent coin... both now and w/ future repair costs.