Assuming that it was given to me legally;
£10 - Keep it.
£100 - Bank it.
£10,000 - Buy the parts to build a better PC, and bank the rest.
£1,000,000 - Buy a HAL-5 exoskeleton, buy the parts to build a better PC, and bank the rest.
If I just found it lying around;
£10 - Keep it.
£100 - Turn it in to the police.
£10,000 - Turn it in to the police.
£1,000,000 - Turn it in to the police.