Virtually any number between $1 and $100/yr seems reasonable to me. Consider: for the rare (and hopefully rarer and rarer) cases where you need a "native" IP address, 1 will almost always do. Meanwhile, all these prices are feasible (in a first-world kind of way) for individuals, but untenable for wasteful use by company IT departments.
$6.5MM/yr may not sound like a dealbreaker for a F-500's /16, but that's actually quite a lot of headcount.
Unfortunately $25k is quite a lot for a /24, and you need a /24 if you want to have a network with independent BGP routing (because this is the smallest block in most BGP routers)
I manage the networks for a small company and we only need a few public IPs, but because we want the reliability we have a dual-homed connection via two ISPs, and our own /24.
Most small companies are not in a position to get a BGP-advertised default-free connection to multiple ISPs for an allocation as small as a /24. Can it be done? I'm sure, but only after a sizeable investment of time and effort.
My point being, it's not as if this is a capability IPv4 currently provides that is imminently going to be snuffed out by address exhaustion. The ship sailed on free/easy multihoming when Sprint started filtering anything smaller than a /19, back in '97.
You obviously have more experience with this than me, but in the UK we had no trouble getting BGP advertising with two ISPs for incoming connections (though we're not default-free, just using HSRP between ISPs for outbound).
There are plenty of people who have done this more recently than 1998, which is the last time I had to register an ARIN allocation. That was for a fairly large regional ISP in Chicago, for the sole purposes of multihoming, and it was a nightmare; I ended up having to call in a favor from a friend close to ARIN.
Surely someone who runs a multihomed hosted app (maybe a YC company) can chime in on how easy it is for a startup to get a BGP advertisement in 2010.
Most IP allocations to organisations these days are not PI (provider-independent) blocks from RIRs; they are simply aggregated from an upstream ISP's larger announcement. That doesn't mean you can't punch holes in their aggregate by announcing that same block (if it's a /24 or larger) through ISP #2, but it does not result in any new numbering allocations from ARIN, net.
It's pretty rare to get a PI block these days unless you're pretty dang large, and taking aim for at least a /20.
Well, you have to have your own ASN to announce it, yes. :) And certainly, you have to get the other ISP to allow this announcement from you through their filters, but that's a normal part of establishing BGP routing with another provider.
But the point is that if Level3 announces 4.0.0.0/8, yes, you can announce 4.16.73.0/24 through them, and through Global Crossing as well. Level3 does have to forward your announcement to its peers de-aggregated, though; that is, if they just fold it into their 4.0.0.0/10 announcement, incoming traffic will prefer your /24 announcement through GX because the prefix is longer/more specific, and plus that gives Level3 no way to withdraw the announcement if the link goes down. Not everybody will happily let you punch holes in their nice, clean aggregate like that, potentially hosing their AS as a whole with flap dampening penalties and such if the link is flaky. However, it is normal order of business with Tier 1s.
But yes, if you have an ASN, you can announce a /24 or bigger directly. You can't do anything smaller, though I have the sneaking suspicion that may change. There are some cranky network operators out there who have not upgraded to equipment with the horsepower and RAM (most importantly RAM) to hold a full BGP view consisting of prefixes down to the /24 granularity, and will indeed filter higher, but they're techically misrouting -- their problem. In any case, that's not the norm, no.
If everybody filtered prefixes smaller than /19 or /20, then multihoming would be the province of a relatively small plutocracy of institutions. I haven't run the numbers any time remotely recently, but most announcements, by volume, are of prefixes smaller than /19 for sure.
If I'm reading "Prefix Length Distributions" right, 52% of all announcements are /24s, and average prefix length is 22.33.
Now, technically, what's being announced != what's being filtered by influential backbones. But it has to be pretty dang close, or ~52% of all announced networks would not be routable from Sprint. :-)
