I have an Oral B one, not expensive version, about $25 (Aus). Lasted over 2 years so far, no sign of letting up.
The Oral B heads seem to last a little better, but I can buy packs of heads far cheaper online from Kogan (kinda like an Aussie Amazon I guess with a lot of their own branded stuff). The 8 pack I bought were $15.
I could probably save a smidge by using a non-electric toothbrush. But I think its important to look after your teeth well. Studies suggest if you are diligent and take the time that you can do just as well manually, but, I find I do a better job with the electric (esp if I'm pressed for time), so for me, electric it is. I take a manual brush camping/hiking.
I manually floss at the moment, have been solidly for about 4 years now (took a long time to develop the positive habit). When my current large spool of floss runs out, I'm going to invest in a water flosser. I struggle to get to my molars, and water flossers do a better job.
If this electric expense saves on some dental work at least once then it will pay for itself. Keeping your teeth pristine helps with your life expectancy too. Well worth it :-)