First of all only see an endocrinologist who has an interest in the area of reproductive endocrinology.
There are many fields of endocrinology/many types of endocrinologist that do not full understand the issues involved with the hormones that require testing in a man with gynecomastia.
You would need a bare minimum of the following tested;
LH
Fee Testosterone or total testosterone in combination with SHBG.
Prolactin
Estradiol (sensitive assay for men if possible).
It is also a good idea if you can get TSH tested as a basic thyroid test.
If you are reasonably well in yourself and only have the gynecomastia, that should cover matters.
If you are run down and ill then liver and kidney tests should be run and additional tests maybe required to further ascertain any problem you may have.
As for cost.
Tell me the nearest city to you and what state it is in and i'll get you some contact details so you can phone one up.