I had a cheap one too and it was fine. I have seen some people warn against the cheap ones as if they get punctured they deflate with a pop as opposed to a slow leak, which you might not want if you were sitting on top of it at the time! But how likely is it in reality to be punctured? One tip - get a nice big sarong or scarf and knot it around the ball, like a cover. Makes it much nicer to lean on especially if you are resting your face on it (all fours type position) as bare skin on vinyl can be a bit sweaty and unpleasant!
birthing balls are often bigger than exercise balls - these are the recommended sizes (found this [[ http://www.tens.co.uk/acatalog/TeNSUK_BirthBalls.html here]]) - FROM £12.95 for a decent one
If your height is up to 155cms (5' 2") we recommend a 55cm ball. Between 155 and 170 cms (5' 2" - 5' 8") you should use a 65cm ball and if you are taller than 170 cms the 75 cm ball is the most appropriate.