heh...you can buy anyone of those entry level bike non brand type you see alot of people riding about. Cost about 300+.. heavy some more so good work out for your legs heheheh...The parts should last you quite a few years as it wear down depending on how hardworking you are at cleaning and servicing the parts. If you really going to placeyour bike outside your flat then you should not buy anything expensive above those cheap bikes.
To me a really "decent" bike starts at around $1000 to $1500 for something reliable,quite light frame, relatively light and better components. Price sweet spot will be in the $2500 to $3000 for something pretty impressive already.
Anything after that is very incremental improvement and you are really paying for exclusivity, asthetic, reputation of the brands, exotic parts and a little more durability due to using light & strong material like carbon fibre or amalgamated aluminium eg.and of course the latest cutting edge tech....just like cameras.
Biking for just some toning of your legs and get some good workout for your bodydoes not require anything then just a $200 to 300 bike heh.. I have seen people on old heavy rusted bicycle beating more yonger fit guys going up hill on their exotic bikes. heheh Anything more to pay for is about how passionate you are with the sport. Again...just like Photography...... your choice.
In Photography you don't jump to a D300 first ....you start off with a D40 for example and as you get more sure this is what you want and your ability improves then you opt later to buy a D300 or D3. Bike is no difference leh.
You might consider a second hand mid range bike....if you want to get more bang for your bucks. There people out there who change bike or parts like nobody business but their vanity. Why I have no idea since no girl is really impress with a guy on a bike. Anyway...you can take advantage of such folks who are selling when they upgrade. (similar with photographing hobbist too right? heheh)
You can sometime get yourself some great bike or parts for less then half their price and just a few months old. Some are very well looked after too. And some of these parts can last a nuclear attack before it even start to take damage while you are fried by radiation. heh.
www.togoparts.com is a good place to start. You get to see anything from cheap practical rigs to exotic numbers there.
Anything else you want to know ....you know where to find me lah bro.