It is possible to get a rough estimate of how much a click would cost for a list of keywords. You can use Keyword Planner to get this data and do the budget calculation manually. However, please keep in mind that cost per click suggested by Keyword Planner is a rough estimate and varies with actual cost.
I'd suggest running the campaign with a minimum budget, to understand the trend for a day or two, so you can calculate how much you'll have spend, to get 300 clicks.
Hope that helps!
Then you can calculate your budget.
CPC * 300 clicks.
Please limit cost per click not to overspend.
P.S. you will spend less because of auction system in adwords
As Sumanth mentioned above, I would like to add here that any tools and techniques you use (whether Keyword Planner or any 3rd party tool), the data you get is only approximation. You can use that approximate data to fine tune your campaigns.
Also, it is best to choose what budget you or your client is comfortable spending on AdWords because there's no minimum limit. Based on that, you can draft your campaigns inside the AdWords account and work accordingly.
It is better to focus on quality of the clicks and not on the quantity so as to achieve relevant results for your account.
My 2 cents!