Yeh that's it.

If you want to convert:

For an underdog, take the american price, divide by 100 and add 1. E.g. +200 turns into $3.00 (200 divided by 100 = 2.00, +1 = 3.00). You'll see the pattern
+200 is $3.00
+300 is $4.00
+240 is $3.40
+500 is $6.00

For favourites:
convert american to decimal, divide 100 by the american odds and add 1. E.g. say the odds are -200. 100 divided by 200 is 0.5, add 1 = 1.50. You can usually do a rough estimate just by looking at it.

The exact formulae (where P = odds)
Underdogs: P/100+1
Faves: -100/P+1