One of the things that makes Block Constructed so interesting is that every block plays by slightly different rules. In Standard, you have staples like Counterspell and Wrath of God and all the rest, forcing decks to sleeker and tighter designs. New strategies emerge and countermeasures are devised, but there are some things that just can't be done, decktypes that are invalidated by the large card pool.
In Block, those barriers are removed, and whole new vistas are opened. The key is to learn what rules govern the block, and take full advantage of them. In Onslaught Block, it's the complete lack of disruption. No reasonably priced countermagic, no Duress, no Stone Rain. If you have a plan, you'll be allowed to execute it. As a result, cards that cost more than you'd expect are seeing play, just because they perform necessary functions. Cost is much less of an issue. Here are some of the effects that players are willing to pay top dollar for.
Players were willing to plunk down six for Wrath without even batting an eyelash. They even scored the double-edged functionality of Purify, and cycling to let it act as point removal with a Lightning Rift in play. Still, six mana is a lot, coming a full two turns later that the original board sweeper. All the dangerous decks are packing acceleration, so getting there can be a struggle. It starts to look even costlier when you realize that Destroy All Monsters really is just that. Silvos, Rogue Elemental laughs and readies an eight-point rejoinder.
What am I bid on the lowly Llanowar Elf? In an environment with overall inflated costs, good mana acceleration also must come at a premium. Frankly, the players escaped this one with their shirt. They'd probably have gone down to an 0/1 with an upkeep of "Pay one life". The extra cost doesn't really matter considering that the spells you really want to play all start at four and not three.
You want Rampant Growth? We'll give you two, but you have to buy both. The on-switch for many of the best decks, The Vegetables aren't even limited to acceleration. Kai Budde is running a four-color concoction that "splashes" Akroma's Vengeance. It's a spell that lets you play with the best spells, but it's not like it could cost any more. At five mana it'd let you have eight on your sixth turn. Not really worth dancing in the streets. Not like turn four Silvos.
Exalted Angel getting you down? Silvos regenerating just a little too much? Need an instant-speed answer to the Rorix that's going to steal the game? Players are in such dire need of Terror-effects that they've gone all the way to five mana, ignoring the second clause of the spell entirely. Smother's stock drops significantly when the big boys come out to play.
Force Spike? Fresh out. Force Void on the other hand, that you can have in abundance. We're giving them away! We'll even throw in a slightly used Mana Leak. Another consequence of the emphasis on big, splashy effects, Complicate is a very timely answer to Explosive Vegetation. It also manages to catch a Pit Fighter here and there, and when you're countering something that big, who cares if you get to draw a card or not?
Skirk Fire Marshal
Protection from Red? Oh, interested in the Goblin mirror-match, are you? Strange as may sound, the Swedes have all placed their stock in the five-mana ogre. With Goblin Burrows he's an almost-unanswerable four or even six damage a turn. He can also play a mean defensive game. Activating his ability is not on the agenda.
Of course it's not all overpaying. The inflated cost of most of the block has a lot of players scrambling to find deals, and there are a few real gems. Pacifism is practically as good as Terror, even answering the mighty Akroma. Less than stellar against Constested Cliffs, but you can't have it all. Vitality Charm lets the beast decks get around Starstorm and Akroma's Wrath. Silklash Spider has been frustrating creatures all weekend, stopping Rorix and Akroma cold. Eight mana just isn't all it's cracked up to be. Daru Sanctifier is a key weapon against Astral Slide, and the 1/4 body on turn three is a godsend against goblins. Lastly, there's Oblation. As outrageous as it sounds, some are touting this as the big tempo card. When everyone's got a handful of six casting-cost spells, what's the harm in handing them two more?