You get what you pay for. At least most the time. Hitachi is the best hands down. Some say too strong, but there is a low setting. Drawback is it does have a power cord. However the cordless ones are weaker and could run weak before you are done. If you spend money on cheaper ones, eventually the money could add up before you find what you like. Bite the dollar, get an Hitachi. Mine has lasted more than 13 years now. And I will get a new one if this one stops. Mine gets used a lot too.
Save your money for one if nobody will buy you one. It will be worth it. btw, be sure you have privacy when you use it. It is not the quietest product on the market.