I think there's a combination of things here, but it's not the printer/razor model. You just don't have to replace the batteries frequently enough for that model. The battery pack should last for years. If you can cut your lawn in one charge and you cut it once a week, you should get around a decade out of it (assuming 500 charge cycles, LiIon batteries are usually 400-1,200 cycles).

The battery you're looking at isn't just shrink-wrapped cells. I've seen 1-kWh e-bike batteries around $150 on AliExpress (shipped from China) that are just shrink-wrapped cells with a cable coming out where you're supposed to be knowledgable enough about batteries to make sure you use it safely and not catch on fire and such. This is in contrast to nicely packaged batteries that cost significantly more (like $800 for 1kWh at Luna Cycle).

This battery is available when you want it (rather than shipped from China with a month wait), you're paying for the retail availability and service/support/returns, you're paying for knowing that the cells are likely reputable cells from a major manufacturer like Panasonic, LG, or Samsung, and you're maybe paying a little for the fact that it's a product-specific adaptation to fit the lawnmower.

However, it doesn't look like they're marking you up much at all. E-bike batteries are a good comparison here. E-bike batteries are a relatively competitive market and the pricing sounds about right for something that's supported, has US distribution, decent quality, not a fire hazard, etc. - $350 for 420kWh. It doesn't really sound like you're getting over-charged. If you want a battery from a well-priced but reputable source, it's not $150 per kWh for batteries of this size. It sounds like they're charging you about what batteries of that size go for. You can certainly search around for ebike batteries and see that despite their being a competitive market there, you're not paying $150 per kWh. Doing a little searching, I can see 1 kWh for $800, 850 Wh for $620, 900 Wh for $600, etc. On Alibaba, I can see some cheaper things, but we're still talking $230 for 600 Wh, $400 for 900 Wh and who knows when they'll arrive or what quality they'll be. The lawn mower company might have a little margin on you, but they aren't over-charging a lot. From what I've seen for US-source e-bike batteries, it's pretty much in-line with my expectations. I've seen 420 Wh batteries a little cheaper (like $275 is the cheapest I've seen) which is cheaper, but you're looking at a slightly more niche product (a battery for a specific lawnmower) and likely looking at a bit more retail markup (like a big box store vs. a no-name online retailer), etc. $350 is pretty competitive and you're going to need to spend it once a decade so the possible $75 margin doesn't seem like much at $7.50 per year.

I mean, you could maybe buy an e-bike battery and get it to work with the lawnmower, but I don't see e-bike batteries that are that much better for pricing. I mean, if you buy the lawnmower and it lasts you a decade and you love it, I don't feel like a $75 markup for a second decade on it feels like taking advantage of you. That just feels like retail markup and, frankly, I'd probably trust Luna Cycle more than the retailer I'd never heard of with the $275 420Wh battery. Luna's $460 650 Wh battery would cost the same as a $298 420 Wh battery, assuming the same price per Wh. $50 just doesn't seem bad.