True, very true. Let's take guesses and see where the numbers fall. I'll assume the average depth of discharge is around 70% and, I'll ignore the capacity drop as it ages in the calculations - As long as this error is consistent, the results will remain comparable. I'll be using the unit "Wh/dollar" and a higher number is better.
SLA
I last purchased 48V "12Ah" lead batteries at nearly $100. Under my use with a 20 amp controller, they delivered about 7 AH before reaching cut-off. Since it's projected they would've lasted about 200 cycles before reaching 85%, that's 200cycles*7Ah*44V = 61.6 kWh of useful energy delivered over their lifetime. 61.6 kWh/100 = 616 Wh/dollar. (Or, about 20-30 miles per dollar assuming something like 20 to 30 wh / mile.)
LiFePO4
Ping lifepo4 - 48V at 10Ah is about $420 at current, including shipping. They will probably last about 1000 cycles before dropping down to 85% (And the newer ones might even last longer since they have a better internal resistance than the older ones). That's 48V*10Ah*1000 = 480 kWh. 480kWh/$420 = 1142 Wh/dollar. (Or about 39-57 per dollar assuming something like 20 to 30 wh /mile.)
NiCad
Jeez, I haven't found the best pricing for this chemistry as I've largely ignored it. But I'll take ebikes.ca's pricing and take 70% of that to find the price of "2C" nicad, to make the chemistry's performance comparable to the batteries I've calculated thus-far. Anyways, it appears 48V 8Ah 5C nicad is $350 from ebikes.ca, which includes shipping. Taking 70% of that is equal to about $250. It seems their cycle life is also around 1000? So, 1000*48V*8Ah = 384 kWh. 384 kWh/$250 = 1536 wh/dollar. Using ebikes.ca's pricing, this would be around 1090 wh/dollar. Also, their longevity is proven by history. (At the hypothetical 2C rate and price, that's about 51-77 miles per dollar.)
Ok, based on the projections, it appears lifepo4's long-term pricing is about 2 times as cheaper in the long-run than SLA, but if you can find cheaper "lower rate" nicads that aren't crap, then it seems like comparable performance can be had at a lower long run price. But, this assumes that you'll use the lithium on a regular enough basis to see its 1000 cycle life before its "calendar life" takes a heavy toll on it, and the same with the NiCad. The break-even point, for lithium being compared to SLA, is when lifepo4 lasts twice as less than projected (500 cycles to 85% of original capacity), but I don't think we have had evidence of that kind, yet, and I'm pretty sure it would exist if that were the case.
It appears the provable long-run operating costs of 5C NiCad is equivalent to the unproven long-run operating costs of 2C LiFePO4, and it appears that "lower C" quality nicad can be had cheaper. So, on the basis of long term operating costs alone, it seems like Nicad is the best. But then there's headways cells and those are a little cheaper than ping(Taking into account costs like a BMS and time for construction - essentially, building costs.) but for practical purposes, it seems they're nearly equivalent and if they last 1000 cycles, then that would suggest that the 5C ebikes.ca nicad has a nearly equivalent long-run operating cost as the headways and they're comparable in performance.
So, apples for apples, it appears higher performance 5c nicad and lifepo4 are equivalent although the lifepo4's lifespan from Headway remains unproven.
So, at 5C, nicad seems to be a safe choice although if lightness and newer unproven technology is your thing, then lifepo4 seems like a good choice. At 2C, nicad would probably be significantly cheaper but I'm not sure if "2C nicad" exists and near the price levels I predicted. But, if I were a buyer for 2C technology, I would most definitely choose nicad after this assessment assuming my assumptions are almost accurate. The fact I chose 2c lifepo4 probably was more due to my ignorance and the "herd mentality" that I was more vulnerable to as a beginner. Anyways, after this assessment, I have to wonder - Why isn't NiCad more talked about?
What is attracting me the most is the idea of being able to buy the NiCad "D" batteries first (which have a long life) and then upgrade to some NiMh "D"'s later and since I'll be making two nearly identical bikes the tubes and batteries can be swapped back and forth. I'd be sure to get chargers (4) that would be able to charge either type. Seems the most "flexible" and "affordable" of the solutions.
