Running Shoes

Hey, 

Ive had some running shoes for a while. However, I have noticed that I am easily able to fit my fingers inbetween my heel and the end of the shoe. 

Does this mean the shoes I have are too big? also, if they are, does it have much of an impact?

If you recommend buying new shoes, what shoes would you recommend? Because im a student id ideally like cheap ones. However, is it worth paying that little bit more? 

Thanks for your help!!