This is your classic "how long is a piece of string" question, but the good news is this: you don't have to spend a fortune for vastly improved sound.
Any of the models on the list above will give you vastly superior audio to that which you'll receive when simply playing through your laptop's speakers, say. Plus, it's a sound that will go much louder than your laptop or PC can manage, with far more depth, breadth and detail to boot. Bear in mind also that, because we've picked hi-fi level units, you'll be able to use any of the above candidates outside of a computer-based use case.
The best thing you can do, once you've decided on a budget (or even to help you decide on one in the first place) is to head to a proper dealership and listen to some speakers, starting at the lower end (see the Q Acoustics M20) and working your way up. If all you need is more punch, power and volume, the M20 will do a stellar job of providing everything that you need to suit your needs.
There's no limit to how much you can spend, but just bear in mind what you'll be using your speakers for. Paying thousands for a pair that will be playing from a crummy audio source (YouTube, for instance) is pointless, but if you're planning on using them a lot and for various tasks and duties, it can definitely be worth splashing a little more cash.