As with all things, you get what you pay for. There are varying tiers when it comes to tablets as far as price/quality is concerned.
HP's webOS tablet is the cheapest mainly because they're no longer manufacturing them and I don't believe developers are releasing new apps/patches due to the death of that operating system's manufacturer support. That being said, many are working diligently to make Android's OS work on that tablet. Many have successfully ported the OS but haven't been able to make the touchscreen work properly.
As far as cheapest functional tablet, I would have to give that to the Kindle Fire - very inexpensive price point and it does exactly what they set out to do: it's an e-reader with app access. Not all Android apps will run on it but it does a good job with the ones that have been developed and it's rather quick. That being said, it's still a fairly limited device.
There are hundred and hundred of Android tablets on the market in varying states of pricing and feature sets. Right now, I would say the best ones are the Samsung Galaxy Tab, the Toshiba Thrive or the ASUS Transformer Prime. If I were selecting an Android Tablet, I would get the TF Prime - sleek design, great performance from quad core processor and when paired with the keyboard dock it has fantastic battery life. That being said, I have quite gotten the feel and join of Android that many of my friends have. This is entirely because my first smart phone was an iPhone.
For an all-around tablet, I think the iPad is still the best on the market. They have really made every effort to do it right when it came to the table - they have a huge developer community (lots of apps), they have great hardware (best screen on a tablet, still one of the quickest feeling processors/gpus and great battery life) and lots of accessories on the market. If price is a concern, I might even recommend looking at an iPad2 - Apple sells refurbished iPad2's and they give you a fresh battery and new exterior pieces on those devices so it's just like getting a brand new one except it doesn't come in a white box with Apple on the side.
I will be the first to admit, I am not a fan of Mac computers but Apple really has won me over with their iOS devices (iPod, iPhone & iPad). I didn't want to like them but I gave them a chance and I can honestly say it will be very difficult if I ever have to transition to a different type of mobile device. I would look at features that you want in a tablet and look at what you would like to be able to do with your tablet and then pick the device based on pricing and what meets your needs.
No matter what you pick, I would recommend going into a brick and mortar store to compare it against other tablets - you'll feel more secure in your choice if you pick one up and fiddle with it.