Lower flex coupler for steering column

Your trade-in value is around $5,000-$6,000 USD if your car is in fair condition and if you live in the U.S.

It would most likely be in your best interest to have it fixed for the $400 USD. But before you spend the money check your belts and fluid levels. If your power streering belt broke this could cause your problem and can be fixed for $20 or your power steering fluid could be low, this would be a $5 fix by adding fluid. If you are not sure what you are looking for ask a mechanically inclined friend for help.

Kelly Blue Book defines "Fair" condition as: "Fair" condition means that the vehicle has some mechanical or cosmetic defects and needs servicing but is still in reasonable running condition. This vehicle has a clean title history, the paint, body and/or interior need work performed by a professional. The tires may need to be replaced. There may be some repairable rust damage.

If the steering wheel does hardly move, it means that the hydraulic steering system has no more fluid in it. If the only problem is the lower flex coupler, I would have it repaired... but not for 400 bucks!

Definitely fix it. Whoever you trade it to will have to deduct more than the repair cost for risk. But get a second and third diagnosis and estimate: $400 seems expensive.