Simple common sense:
- If you rely on your PC you need a UPS
- A 700W UPS costs £600
- Laptop has UPS built-in
Not sure where you buy them, but a standard APC UPS costs between 99 and 150 euros here, depending on where you buy it and on how many Watts (anywhere from 500 to 800) you want. That's much closer to £100 than to £600.
Replacing the UPS battery after 3 years costs around 42.80€ at Amazon (original manufacturer parts). I could probably find them cheaper too, but at that price it's not worth wasting my time looking.
Replacing the laptop battery (again, using original manufacturer parts) costs around 120€. Of course you can get third-party laptop replacement batteries for around 40-50€, too -- but you must not compare apples and oranges.
Also, laptop batteries (at least the ones I've had) tend to actually break after 2 years, or after being through airport controls 6-8 times (no idea what the hell they're doing to them!). I replace my UPS batteries every 3 years because .... well, because they're 3 years old. But actually, I'm replacing them without an urgent reason. I still have to see one failing.