Well I believe science fields is usually the most lucrative in the research field.
I have heard recently from my PHD friend in chemistry that Nanotechnology is actually receiving significant amount of grant money and is the in thing as far as research is involved. Obviously any application to do with stem cell research (toxicology) is another very in field at the moment. In addition pharmaceutical company research do pay quite well also.

But just to paraphrase this excellent yahoo answers post i think it may also apply to other fields, if you disagree by all means please post your answer below.

"The medical field is the highest paying jobs that have a biology and chemistry science fields involved with them.

Chemical and Materials Engineering also pay well but not as well as the medical field unless you get into Biomedical Engineering.

"Environmental Engineering and Environmental Science is not that well paid in relation to the medical field and the Chemical and Materials Engineering.

Don't go into Zoology or Marine Biology because a McDonald's manager gets paid more than a person with a Bachelors in those two fields. Only go into those fields if you really love it and don't mind working for nothing and want to get your PhD so you can earn a somewhat decent salary to live off of."


http://answers.yahoo.com/question/index?qid=20071223141413AAoXQoF