You know.. you don't really need all that much cash on hand to start with media buying.
You can easily start with $10k-$20k and be able to score $100k-$1M media buys in under 6 months without a doubt. Unlike PPC for most people, media buys rely heavily on credit terms. If you call most places up and play it cool and tell them you want to drop $10k-$20k on a test campaign, they'd probably offer you credit terms right afterwards. Just make sure to build up credit with your business/LLC/Corp first.
Getting approved for a media buy campaign with any major company like AOL, IAC, Google, Yahoo, MSN, FOX, etc. gives you instant credit for future campaigns with just about anyone in the space. They see anyone of those companies as a reference they'll issue you an almost limitless amount of credit terms.
Hell, just in March we had only $300k/mo credit terms with most companies, and now we've got well over $20M/mo - unlimited, at almost every network I've mentioned above, not that we'll ever even use it or come anywhere near close to those, but it just goes to show you how much they'll offer you just so long as you can get the references. Its totally worth it to drop $10k-$20k pre-paid in your first test month and be in the millions in under 6 months.
Best part of credit terms too, is that almost every outlet you buy media from is payment on net-60, some of them will go as far as net-90, but you may have to blow the right people for that. That means you pay 60-90 days AFTER the campaign runs. Gives you ample time to collect your earnings and pay your bill on time without any issues.
I know I'm making it sound easier than it is, because the toughest part of media buying is not being able to find the space or even buy the space, its actually figuring out which places will convert at 300%-1000% ROI vs the ones that only convert at crappy 50%-80%. Thats the difference you find in the industry these days between the punk kids who go around yelling about how they do media buys but can never quite figure out how to surpass the 300%+ ROI point. You'll never hear or see anyone post the answers to that kind of info here or anywhere though, unless they are on crack or lying. But yeah, there's no reason to give up and walk away just yet buddy.
The smaller ones are good too sometimes. Just depends on the offer and what it calls for, either to be pounded with general media buys from unfiltered/untested sources, or from well planned out purchases that are more expensive but proven to work. Tatto Media are good people, almost limitless inventory too, known them for a long time now, definitely take a stab at them. The others you mentioned are decent too.
Almost all of the 2nd tier media traffic sources are also affiliates. CPX and ValueClick are both a huge affiliates and place a lot of guys here buy media from. Hell, we caught those fuckers stealing our ads/LP's before, so just goes to show ya, you may think its a safe bet to buy from but the risk is always there. Just because they sell traffic, doesn't mean they aren't in it for themselves too. But thats just the way the business works.