The number of RDs is a helpful indicator taking all other things equal, but if you're looking to rank, increasing RDs in itself shouldn't be a goal.
1 link from the BBC will be better than 20 links from profile pages. You want to assess the ratio of inbound to outbound links from each page linking to your site.
If you want RDs, then comments, profiles or press release would all be cheap. They won't push your rank.
If you want to rank more highly, get a guest post.
@Kuz32 has some well priced ones.
EDIT: If you're looking at whether you could beat another site, then a quick way to do it would be to look at how much it would cost to buy a much more powerful domain. I look at how much it would cost to buy a domain with 2 or 3 times the number of RD.
No link tracking software picks up all RDs, and they certainly don't if the site I'm trying to beat uses PBNs.
For example, I'd reckon on buying a domain for $500 - $600 to make sure I could beat a site with 100RD. You also need to factor in hosting costs and good content - its not all about links.
There'll be people saying you can buy a cheaper domain that will work, and you probably can. If I'm going to the effort of writing the content, I'd rather overdo it than underdo it.