It depend how much does the keywords cost per click . Usually popular keywords , high search keywords , wide keywords will cost more to you.
if there are only 320 monthly searches, your impressions and click through rate will be very less. try to use more broad based keywords in your campaign. However, to answer your question- the cpc varies between your target markets. If you are targeting united states, it may cost you around 5 cents. You can create the ad and check the bid prices to see the actual cost before activating the campaign.
Its up-to you, you can set the limit to what ever you wan't, but then it depend on the keyword price that your add will show or not.
you can estimate the cost per click in your adwords account, didn't you notice it ? go read the whole google help under adwords.