Does 多 always come after the measure word？ I've realized that I still don't fully understand the placement of "多“  a relatively common word. Can you never place 多 before a measure word? 

A friend recently corrected my "四多快美金“ into 四块多美金” but does that apply for all measure word 
比如， 三十多条蛇 should be "三十条多蛇？“ (that doesn't sound right either) 

Any clarification would be super helpful!
多 is used for approximate number, which is one-tenth of the measure unit prior to 多.

四块多美金 means 4.0-4.9 dollars. 四多块美金 is not correct, 多 should be placed after the measure word 块 for unit digit.

三十多条蛇 means 30-39 snakes, and 三十条多蛇 is not correct, because the measure unit of 蛇 could not be decimal.