They do indeed outperform SLA and are worth the money if you can afford the upfront cost and have a solid BMS system to keep it safe. With the SLA at 48v there is the option of using audio super caps to squeeze every last bit of juice out of SLA batteries and keep the voltage up but that also adds weight and the SLA is about 3 times heavier than Li equivalents so I wouldn't recommend a lot of lead batteries on a bicycle; 36v 12ah would be about 22lb but more powerful packs get heavy. I hope the cost comes down so that a 60v pack of lifepo4 than can deliver 100a continuous would cost about $200 which would be about twice the cost of SLA. I'm bunny hopping my motard and it would be great to shave off about 25 lbs from the bike, lol
Keeping it low power you can have a reasonably priced electric from many manufacturers of kits and bikes, most of the money is in the copper and neodymium in the motors and the batteries of course. Supposedly a large land cache of lithium has been discovered recently, but I doubt that it will affect the market cost of the mineral. Mainly due to speculation and market manipulation due to its inherent value for the electric vehicle market. Electrics can be louder than gas like my monster or silent and smooth like a sine wave hub at low power, I personally like the ruckus nature of a lot of electric power because the acceleration is like no other! Electric is excellent and affordability needs to be focused on so everyone can enjoy it. It will be great when someday a full motor, controller, and lightweight battery can be the cost of the engine kits. At which point you might consider a gas bike obsolete, not in functionality, but because of a social or political stigma assigned to it by learned association. I don't think this will ever happen as long as dirt bike and motorcycle road racing exist and people are out riding their Harley Davidsons and Nortons and the like, gas bikes will be welcome. If you don't like the smell and noise get a long poo catted pipe, run the bike stoich with sweet smelling synthetic 100:1 oil, use a gas tank that seals with a proper vent tube, and you are set. Of course you still can't ride any motorized bike on bike trails which are designated for no motorized vehicles, for the obvious reasons.