I say 10 billion and 10 million. 10 billions and 10 millions doesn't seem right to me.

10 million and 10 billion is right. The following rules apply to the plurals of numerical terms such as dozen, score, hundred, thousand, million, and similar: 1) When modified by a number, the plural has no -s added. Hence one hundred, two million, four score, etc. (The resulting quantitative expressions are treated as numbers, in that they can modify nouns directly: e.g. three dozen eggs, although ''of'' is used before pronouns or definite noun phrases: three dozen of them/of those eggs.) 2) When not modified by a number, the plural takes -s as usual, and the resulting expression is not a number (it requires 'of' if modifying a noun): e.g. I have hundreds, dozens of complaints, the thousands of people affected. 3) When the modifier is a vaguer expression of number, either pattern may be followed: several hundred (people) or several hundreds (of people). 4) When the word has a specific meaning rather than being a simple expression of quantity, it is pluralized as an ordinary noun: e.g. Last season he scored eight hundreds [=scores of at least 100 runs]. The same applies to other numbers: My phone number consists of three fives and four sixes.

million and billion are used without the "s", in numerical figures. When used as a measurement in general (without a figure, it can have the "s" at the end... for example: B說：He has millions of dollars！ A問B: How much exactly? B答：10 million.