The hardest thing for a brand or business to do is to filter through the BS and set realistic expectations for your marketing investment and efforts. Obviously when you have a limited budget you can't afford to do everything you could if say you were a massive brand with unlimited dollars.
I wrote previously to start with your goal and work backwards. Net you can look at your labor and capital resources and choose where you think will match your goals best. I then suggest you 'Sandbag' your expectations and work up some numbers below a homerun. Sandbagging is used by direct sales people when they have to give their forecasts to management, they under forecast so the blow away expectations vs underachieve.
Why should you 'Sandbag'? Marketers hype. That is their nature. Whether it be your Agency or a Media Channel they are going to inflate things in their pitch. You will be told how many people they can reach for you, but often be unrealistic. Especially since most can not prove results due to the type of platforms. A TV station can't prove I was watching your commercial vs getting up to pee. A Magazine can not prove I read your ad on page 33. Facebook can't prove I saw the little ad to the side but they can tell you if I clicked.
I like a 30% discount for anything that is unmeasurable and has to be estimated. So make your own adjustments. If the local cable station says for $500 you can reach 100,000 people with a 30 sec spot which is 0.5 cents per viewer cut that by 30% to 70,000 and use a higher cost. Do this for all your media spend. I would rather reach more people than I expect than reach less than I expect.
Use math as well. I have seen Facebook with 850mil accounts and Twitter with over 400million accounts. But when I crunch the activity numbers happening each day they tell a different story. That in the US only 15million people are actively tweeting and only 27mil to 35mil are active on Facebook. Many more log in and just read. So this means you can reach more with 'Paid' advertising like Facebook Ads, but lower your expectations for sharing of any content or reaching them with tweets and posts. That $500 for a Cable TV 30 sec spot might turn out less expensive than Facebook ads or the labor/time to reach them with posts on your Brand page or Tweets.