In these kinds of Biological Fields (Biochemistry, Biotechnology, Biology etc), a PhD is a must if you want to make an average salary. I am saying Average because even after doing PhD in these fields the salary will not be HIGH. You'll most likely end up working in a lab doing some research or work as a Post Doc (after Phd) and that will pay you around $45k-$55k. And the time we are talking is:

Since you've Masters.
PhD= +3 to 4 yrs
Post Doc= +1 to unknown......yrs

So, after At least after 5-6 years you'll make 45k-55k.....Are you ready for this ? This is a bitter reality in fact.

If you choose a different path after PhD, let's say you become an Assistant Professor, that' will pay you about 55k-65k. Again that's not much.
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Now,
If you can land up in Lab with your Master degree which pays 45k-50k, I would say go for it.
Because if you get a Job with MS degree and say you're offered 50k...The time line to complete PhD beyond Master's is at least 3 years.

By that time you'll have made 50K*3=150k

Suppose, you get decide to go for PhD (you'll loose 150k) and you're offered a job that pays 60k (after PhD).
ie. Difference in salary with PhD Compared to MS = 60k-50k==10k and it will take you 15 years (15*10k) to compensate for that 150k You lost just because you went for PhD.
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Another Suggestion,
How about Considering doing Master's in another field such as Bioinformatics, Statistics, Engineering etc.

After Masters with such degrees you'll start making 70k-80k, more than what you'll make with PhD in Life Science.

Bottom Line,

You Decide


Guy_tensed