Upgrading contributor level (CV) tips

Buy a lot of bundles to giveaway (or giveaway your extras from bundles you've bought) and buy games when they're ridiculously cheap and hope they don't get removed from the giveaway list.

~80% steam sales, indiegala happy hours, dailyindiegame, steamground, gogobundle, or even just any bundle in general.

Time efficient: Lots of high value items.
Cast efficient: Those really cheapo bundles, like DIG, or Indie Gala on happy hour, pick the bundles that have the highest contributor value. This is followed by humble monthlies, but these are always a bit of a gamble.

But best thing to do is to not stare yourself blind on contributor value. Just give what you can. And going for high levels is not worth it unless you enjoy the act of giving.

Cost-efficient:
Non-bundled games on Steam that are around 90% off. (They count as bundled when they reach 95%, but since prices in other regions contribute to that, you usually don't want to go closer.) Steam Wallet is also cheaper than real money.
A lot of bundled games from third party sites that often give even better CV than above. Be careful as many of them are not trustworthy. For the sake of advertising I won't mention any names.
Time-efficient:
It's usually not worth it to buy actual bundles since the bundle penalty will make things less cost efficient, but it is the most time efficient way to spam bundles and/or developer keys.

Sadly, the way the CV system works is that premium AAA games are the least efficient in all scenarios, since they are a lot more expensive and not easier to give away than anything else. But they might land you on more whitelists at least?

Crap marketplaces like G2A and Kinguin sometimes have really good deals, like I got copies of Dead Rising 2 for 4 euro each and the game is not bundled.

Use websites like isthereanydeal.com and epicbundle.com

No wonder i have you in my BL.

Humble Monthly is surprisingly efficient - generally they don't quite make the bundle list, but the total CV usually more than 10x the cost.
Daily Indie Games and IndieGala Happy Hour also do quite well.

Lastly, if you really want to stoop and you are able to get them, region specific giveaways can be very cost-effective
oh, before I forget, whenever humble has some kind of developer bundle, that's a great way to make bank. The base software alone could have a sticker price of $100+ and be in the $1 tier

The most time and cost efficient is Humble Bundle Tier 1 packs. They go as high as 15 CV/dollar, although they are usually in the range of 7-8.
Humble Monthly is also a nice one, if you can afford 12 dollars in one payment.
IndieGala used to be the second one. Buy a bundle at launch, then buy one or two more packs at happy hour, and you have 5 or 9 copies of the same game, usually for ~$1.05/bundle.
Nowadays, the DailyIndieGame store is a better alternative, since you can get keys for 3-5 cents on sale, beating IG's 10-cent average. Just don't forget to cross-reference the games to the giveaway-disabled list to avoid nasty surprises. Buying 6-8 copies is still optimal.
A new bundle site called GoGoBundle offers bulk discounts. CV-wise, 10-packs are the best, or you can band together with four-five more CV miners to buy a 50-pack together.
Lastly, there is OtakuMaker. You can buy 5 and 10 packs there. Again, it is a good idea to split up a 10-pack. Just be careful, they often have freebie games.

Finally, if you like risking your money, you can buy really dirt cheap RU/CIS keys for unbundled (as in never being in any bundle or at more than 60% sale) games, sometimes for about 10-15% of their US store price, on G2A. Since they give 100% CV, they have been used excessively on this site for shameless CV grinding by ROW or simply non-CIS people. Keep in mind two things though. One is that every time you buy at G2A, you are funding tax dodgers by buying games from unknown people and of unknown origin; game keys bought form G2A have been revoked and will continue to be the target or revoking. In which case not only you lose the CV, but get a non-received mark on your profile. The second is that even if the keys will be total legit, this kind of behaviour is often frowned upon on this site when people see obvious cross-region CV exploits.