What are the tones for 不 and 一 in the expression 不一定? 不 and 一 both have the property of changing tone to suit the tone of the following character.

This phenomenon is called the tonal sandhi.

What would the correct/standard tone combination for this expression be?

My thoughts are: the 4th tone on ding causes the yi to take a 2nd tone, which causes bu to take a 4th tone.

4-2-4

Is this correct?

For some reason 2-2-4 sounds ok to me too.
Are there other variations on this?
Yes, 4-2-4 is correct. According to the sandhi rule of 一


  在四声前边要读二声


i.e. 一 should be pronounced in the 2nd tone before the 4th tone.

And the sandhi rule of 不


  “不”在一二三声前面不变调，读四声


i.e. 不 doesn't change the tone (the 4th tone) before the 1st/2nd/3rd tone.

BTW: 2-2-4 sounds weird to me.

Reference: 一字与不字的变调问题