Welcome to Fluther.
I can give you a way that will work, will give an accurate measurement, and can be repeated – and it won’t cost you anything but time and the value of your weight in fresh water (if that’s a consideration where you live). But you may require some assistance to get an accurate measure.
In line with @Zaku‘s advice, but maybe a bit easier to accomplish, you can do that in a bathtub, as long as it is big enough for you to immerse yourself completely. What you’ll want to do is fill the tub to an exact level – it doesn’t matter what the level is, as long as you can submerge completely beneath it and not overfill the tub. Then immerse yourself (you can breathe through a snorkel for a minute or so while the levels stabilize) have your helper mark the exact spot on the tub wall where the water level is with you completely submerged. When you get out of the tub you can use some kind of known measure – a gallon jug, for example, to fill the tub from your starting point to the line where you were completely immersed.
Since your body displaces its mass in water and water doesn’t compress, the measured difference in gallons between “start” and “finish” x the weight of water for the measure will give you an accurate – and cheaply repeatable – way to measure your weight.
Depending on temperature (because water does have varying density), 250kg of body mass will displace between 60 – 63 gallons of water. (At “bath” temperature of around 100°F or 30-ish °C – you’d be closer to the 60-gallon mark.)