多 = more (than...) 你多吃一点吧 = Eat more (than usual) 咱门多等一会 = Let's wait a bit more (than we should) 你多问几个问题 = Ask some more questions?? Go on asking the questions? This sentence means you should ask questions, more than usual, not only a few. 你再多问几个问题 means you should go on asking more questions. 我想多认识几个朋友 = I'd like to meet more friends?? This sentence means I'd like to meet a few more friends (than I already have) 我想认识更多的朋友 = I'd like to meet more friends. 我多花了一百块钱 = I've spent more than 100 yuan?? I spent 100 yuan once again? I spent 100 yuan more (than I should) 这条裙子我买亏了，原价200块，打折的时候只要100块，但我是原价买的，所以我多花了100块！ The interesting thing here is that the THAN part is always omitted, because it is always than usual or than we should.

In my opinion,hmm,多=more,many,several,extra...etc. As an adjective/adverb,it has many meanings,in different combinations. For example(you ve listed):我多花了一百块钱=我花了额外的一百块 then 多 equals to extra(I ve spent extra 100 yuan); 你多问几个问题 = (you can )ask some more questions here 多 equals to extra(for you put 多 before the verb 问);你可以问更多的问题（You shall/can ask more questions） 我想多认识几个朋友 = I'd like to meet friends as many as possible, 我想认识更多的朋友= I'd like to meet more friends; Actually,your sentences can be replaced by some other ones,and the only difference is that their tones will be changed.into a weaker or stronger one. Hmm,as for 你多吃一点吧;咱门多等一会,I think your line of thinking is completely right. Sorry,it is in such a mess here.But I hope that my opinion will be of help.:)