This should be a cheap fix - but you can't just tape the thing up.

Yes, you need a "flippy directional thing". Otherwise, wind will blow right down the vent and into your bathroom.

Since this is part of the sale, I'm sure the sellers will want a qualified person to perform the work. If they read about your tape plan, they certainly will insist on a pro.

My state-licensed GC came by yesterday and told me that I can butt it right up to the gable vent and staple it up.

Then he quoted me $200 to do just that (I suppose they'd do a soffit vent) and to vent the hall bath fan through either the roof (7' up) or soffit (4' over).

If I'm going to hire him for the hall bath one, he might as well do the easier master bath gable one. Does $200 seem appropriate for this job?

"Yes, you need a "flippy directional thing". Otherwise, wind will blow right down the vent and into your bathroom. "

Every bathroom fan I have seen already has a baffle installed, but another one at the outlet will not hurt and will help with air leakage back in the vent.

Make sure you use some type of screen (like 1/4 inch hardware cloth) along with the exterior baffle.

I used to watch the starling flip open the baffle and nest in the neighbors vent.
The wings beating on the metal rain hood attracted my attention.

Even a piece of hardware cloth bent to wedge in the hood without obstructing the baffle movement works (the zinc on the cloth wil lcause some corrosion of a zinc hood, but not fast enough to be a real issue).

There is no permanent use for silver tape in the construction of a house.

HVAC foil tape is probably the most common, though not the most efficient, way to seal ducts.

I stopped using the soffits for venting 20 years ago. Not efficient.

"There is no permanent use for silver tape in the construction of a house."

Depends on what you mean by "silver tape."

Cloth based 'duct tape' was never a good product for duct work, and is generally not allowed anymore.

Aluminum foil "silver tape" may not be as good as mastic, but works well. It does need to be rubbed down thoroughly for long term, and then will work very well.

A plastic drywall knife rubs the foil tape down thoroughly and solidly.
On duct board you can see the reinforcement fibers through the tape when it is applied correctly.

Whatever you do to terminate the fans, do make sure your "flexi tubing" has no low spots or bellies in it.

Those will, over time, fill with condensate and tear the pipe away from wherever.

Mavc told you the right way, your contractor tells you his way.

I'd do it mavc's way using solid aluminum ducting in place of the flexi tube.

The correct name for cloth based waterproof grey tape is "DUCK TAPE". Originaly developed during the Korean war to seal Ammo cans. Because of the similarity of pronunciation, many people and some manufacturers get it confused with "DUCT TAPE"
DUCT tape is a metal foil with a high strength adhesive designed to permanently seal heating ducts and cut fingers. It is seldom actualy called DUCT tape these days because so many people get it confused with DUCK tape.

And then there's Duck Tape brand duct tape to capitalize on the common mispronunciation.

"Venting" bathrooms into other parts of the house is a common shortcut of diy ers. The teardown I stupidly moved into features one of those. I shudder when I walk through a Home Depot and think of all the stupidity inching to the checkouts.

The cloth tape for ducts was in use well before the Korean war, and refereed to as duct tape.

Just to tell you where I am coming from on this - I wrote the book! (It's fun to say that.) "Residential Ventilation Handbook" It's been said pretty well already, but bath fans must vent all the way to the outside. Not just near an opening. Use insulated ducting to reduce the possibility of condensation in the ducting. Use 3 screws to attach the ducting to the hood. That will keep the ducting from turning or pulling off. Be careful that the screws don't interfere with backdraft dampers. (Cloth duck tape was made from the same duck material sailor's pants were made from. Not good at all for duct work.) If you can do it, transition from the 4" nozzle on the fan to 6" ducting. The flow will be much better. Point the nozzle of the fan in the direction of the exhaust hood. Air is lazy. It hates to make 180 degree turns.

Here is a link that might be useful: Heyoka Solutions

If I were to get 6" insulated flex duct are there any tips for the type of louver caps or back-draft dampers I should get? something that can go up against a gable vent that already has a screen? I also need to ask if it is bad to have the vent climb roughly 5' up over 3' horizontally to reach the gable vent in the attic?

Tosin, I recommend starting a new post.

What are you venting

Environment?

The discussion will probably be much longer. So in my blog, I mentioned it in detail. It will definitely help you and others. Read Article (Looking forward to valuable feedback from readers)