Price for Monstermakers kit in USA, from Monstermakers.com, is US$35, but you would have to add shipping to that. Amazon lists it at $34.99, but links back to Monstermakers as the seller.
Same price at TheEngineerguy, $35 for MM foam. They also have the GM Foam 1 Qt kit, for $38.67. So GM is slightly more expensive. Looks like the MM may be the cheapest you will get.
I just checked the current price here, $132 Australian dollars ($92.66 US) for the 1 quart kit! It costs 65 pounds sterling (US$ 80.15) in the UK, at TiltMakeup.com. They also sell one called Mouldlife, slightly less at 49.95 pounds for the 1 quart kit, probably made in the UK so it would not be cheaper in the US if available at all. The high price for MM in other countries reflects the cost of shipping the MM foam from the US, and the reverse would apply, if importing into the US.
Many years ago I used Burman foam, but I believe they bought out GM, and that seems to be the one they sell now. GM was pretty much a slightly improved Burman anyway. I prefer MM now, it is a little more forgiving. You can reduce the gelling agent a bit and get more time to get it into the mould, and it is not as likely to fail to gel at all as Burman or GM.
Foam latex is not used as much now, silicone having replaced it for many uses, so there may not be any other alternative brands still in business. It was always a specialised market, and now even more so. I still choose foam latex where the puppet is bulky, so I want it as flexible as possible and to keep the weight down. It's great for monsters, dinosaurs, old wrinkly characters. But where I don't want any fine surface wrinkling like on a child's face, or where it is important that the puppet keeps for years as a display item without rotting away, silicone is better. However I would not call silicone a cheaper alternative. You use more silicone to fill a puppet mould, because it is poured in as a liquid, not whipped up to a higher volume foam.
An alternative, though not really equivalent, might be to use liquid latex as a skin in the mould and then use Flex Foam to fill it out. This is not latex foam, but the softer versions have quite a bit of flexibility. In the end it may not even work out much cheaper.