This is a tricky question to answer because you're asking about the overall strategy and tactics for running a successful PPC campaign and integrating that with your SEO strategy. I'll share with you some personal results and tactics we've used, and some learnings, and hopefully that will help answer your question.
First, some context. We do a lot of SEM and SEO for local services, so I'll be using HVAC as my go to reference point.
1. Brand keywords are super cheap for us. We've always found this to be a good investment as it doesn't normally cost much and if we don't buy those keywords, some of our competitors do.
2. Buying other brands - just as you want to keep competition out of your space, you can invade theirs. Also hasn't every cost us very much money in the past. Much lower conversion rate from these, but every one you snag is a customer won.
3. CPC varies greatly. Especially for local keywords. Different regions have different competition levels it seems. I've seen some guys with a total Cost per Lead of $45. I've also seen it skyrocket to over $100 CPL in a different region.
4. Don't confuse SEO and SEM. There's a big difference. SEM/PPC is referring to paid ads, SEO is the organic listings. They are two separate disciplines that require two separate skill sets, so you shouldn't have any SEOs trying to sell you SEM unless their agency does both.
5. Bid for profitability, not revenue, and you can keep your costs down so you're bringing in leads at an acceptable cost. As long as you've set up a tracking and attribution model that works you shouldn't have any problems making adjustments until your campaign is running smoothly. You can axe the keywords that are costing too much and improve the ones that are bringing solid leads.
Hope that helps. I do recommend doing SEM on top of your SEO. If you're having trouble running a profitable campaign, find an expert you trust and commit a stable budget for a year and see what they can do.