Should one keep spending money on PPC even if it doesn't convert, hoping that in the future they will come back and it will convert? I"m talking about using google adword and other famous one, which has low fraud click.
That's how I see it, with the minimal information you have given. Then you may want to look into if you're using the right keywords, if you have good adcopy, and if you've done right with a few other 100 things. If you've done them all correctly, give it a month or so. If it still won't work for you in terms of conversion, then you should make a decision.
It may be the the product your selling. Eg if it were those 'Credit Card Offers-Low APR, 0% on Balance Transfers' its a very small competitive market. very difficult to convert
But if it gets the visitors it should convert. For example other form of advertisement non PPC, it converts 1/50 , but with ppc its more like 1/150
I have not started using PPC but I am going to start from next month. The idea is that you should have a budget allocated for each month. Track your visitors from adwords using analytics for 3 months. If you think it is worthwhile , go ahead else not. But what I know that it may not be always about conversion. It may be building a brand name which people could recall.