For PR and networking, it's more about your time and effort than cost. You may need some subscriptions for networking or do some research for the PR. But typically the cost here is your time and effort.
For Direct Marketing, at the scale you are likely to be operating, getting names and addresses or email addresses is probably the issue. Companies will sell them to you, but you need to be targeting specific customers and you need to check the quality of the addresses that you buy. Then create the letter / email and fire away. If you can, running a low scale test that you can execute yourself will be good experience before asking a company to do this for you.
For the broadcast marketing, the niche publications will almost certainly have much lower advertising space costs (hundreds of pounds perhaps) versus the national press (thousands or tens of thousands of pounds).
Google will quote the cost of pay per click searching where commonly used terms like 'holiday' will be expensive (pounds per click in some cases) while very specific terms will be a few pence per click.
In my surfing fathers example, I would opt for the surfing magazine as a quick way of testing for interest. Call the main publications to check for advertising rates. I would also check the last few issues of the surfing magazines to see what the themes are and whether there is any PR angle I could take at some point. I would look into creating a very simple web site and setting up a search account. The cost of 'family surfing holidays' being pence per click.