To answer the original question, I would spend up to about $200 for a nice gig bag that offered sufficient protection and had good storage for accessories. Bonus for being able to transport a micro-head.
As far as why I would use a gig bag when a hard case might provide better protection, it comes down to this. For me, gig bags win "hands down". What I mean is, when I use a gig bag, both my hands are free, since the gig bag is strapped over my back like a backpack. This means I can carry an amp, or a cabinet, or a gear bag or stand. Usually means one less trip when loading / unloading. And since the gig bag goes with me on all of the required trips, the likelihood of my bass(es) coming up missing is seriously minimized.
This does mean that I have to take extra care to not put my bass(es) in a situation to experience damage that might not have been an issue with a hard case. In my experience, the only such concern is in how the vehicle is packed. I have to make certain the gig bag is not in a situation where anything can fall on it. Very easily done.
When I travel by plane, I take a gig bag. Every time I have taken a hard case, I've had to check my bass. All but one time I've been able to carry it on with a gig bag. That one time they gate checked it and, as I found out after the flight, simply put it in the flight attendants' closet.