how much a good bike can cost?
As other people said this is difficult to answer within the subjective field of "decent" "good quality", etc.

There are two main components to the price of bike(part)s:


Weight (lighter is generaly more expensize)
Quality (which is a catch-all phrase for durability, the technical sophistication needed to make a part, complexity of the part, extra effort made to enhance comfort, etc. anything other than lowering weight.)


Increasing price will get you a bit of either, or mostly the first or the second, depending on what the supplier of your bike considers important. From your question I think that you want to focus on quality more than weight, so don't go spending on carbon, titanium, and other exotic stuff. This rules out most bikes above about $2000 - $3000, so there is your upper limit.
I'm not sure what you expect from your bike in terms of features, and when and how you'd expect to use it.

You can get a pretty decent quality no-frills single speed bike for the price you paid for your bike, but it will be bare. No fenders, lights, no chainguard, no luggage rack, coaster brake.

If need more than that the price starts to increase, and all I can say is: "it depends" but you'll stay within the upper limit I gave above even if your definition of "decent" can compete with my "very good" as long as you mostly ignore weight.

To give you one last pricepoint: For a decent daily use commuter bike (15k one way, mostly flat, wet/cold winters, full time riding) I need fenders, a rear luggage rack, a full range of gears, lights, leak-proof tires, low maintenance- high durability components, a comfortable saddle, mountainbike handlebars with bar ends, and very good and dependable brakes. I would expect to pay about $800 $1000 for this bike. I would expect to ride this bike for 5+ years, only replacing brake pads, tires, chain, chainrings and cogs, and maybe the occasional brake- or shifter cable, or spoke.