Current retail for a 1928 gold certificate ranges from $250 to $750 (retail). I suspect that yours might be towards the higher end if it has minimal wear.
Also, as you've seen from other postings, a bill's serial number is almost never a factor in its value. Seal color, date, series letter (if present) and condition are the most important things to provide in a question.
Its worth at least a dollar!
Washintons head folded is the mushroom!
it is the ribosome
The mitochondria
It depends on how much wear it shows. However, in any case $50 notes didn't circulate a lot in those days so despite its age, a folded bill with that date, regardless of series letter, wouldn't retail for much more than $65; less if there's significant wear.
It is endoplasmic reticulum.
A 1921 silver dollar with the eagle's wings are folded would be a Peace dollar. This design was put into production at the end of that year and is much less common than the 1921 Morgan-design coin. Because of its relative rarity, any apparent 1921 Peace dollar should be inspected by a professional to determine its authenticity.
The coin was issued to honor the peace following WW1. The eagle's wings are folded because it's at rest and symbolizes the country's mood.
Endoplasmic Reticulum
Folded mountains form when two plates move towards each other by compressional forces. The movement results in sedimentary rocks moving upwards to form a series of folds.
mitochondria
The serial number of a bill is almost never a determinant of its value. You need to provide the bill's date and, if it exists, the series letter (a single letter after the date). Also, how worn is it? Just folded, torn anywhere, or is it crisp and unfolded?