Sunglasses are required by law to block UV, in Australia at least. Even most untinted safety glasses (with "O" outdoor marking) have UV blocking in Australia. Another option cheaper than laser googles are welding googles/lenses which are good enough for low power lasers due to their need to block intense UV radiation and visible light from welding.
The problem is that 405 nm doesn't really count as UV, it is just a deep purple... You really have to look at the wavelength, you can't just assume that just because it says UV it will block a laser (and are we talking about UV-A, B or C?). I'm willing to bet that sunglasses don't provide protection against 405nm, just because that would distort the colours. As you have already mentioned, the transmittance increases quite a bit with the wavelength, if you have 50%@365nm, then what do you have at 405? And let me just quote the product safety page of the australian government regarding UV blocking sunglasses:
The mandatory standard requires that sunglasses must protect users from ultraviolet radiation (UV) within the range of 280 and 400 nanometres.
405 > 400 nm... And 5 nm can make quite the difference for laser goggles as they tend to have rather sharp absorption edges.
But let's tackle the most important reason why both sunglasses and welding goggles are NOT suitable for lasers:
LASERS EMIT COHERENT LIGHTBoth the sun and the arc from a welder won't do that and thus won't even come near the obscene photon/energy densities of a "small" 100-500 mW laser. Combine that with the fact that blue (and probably 405nm) light is by far the most dangerous wavelength for your eyes as it will instantly boil the blood in your retina -> not great. Just to really pronounce the difference between a laser and a "normal lightsource": If i project the full output of my 300 W 30 mm mrad laser at a wall 10m away i will get a spot with a power density of 240 kW/m^2. If i switch on a 300 W lightbulb i will get 1 W/m^2... Same power, yet the laser provides a spot with 10^5 times the power density. The intensity of non-coherent sources correlate with the distance cubed, but lasers don't follow that rule... In that example, the 40mm spot from 10m away just becomes a 130mm spot at 100m.
Blue 445 and 405 nm are just on the edge where the lens of the eye is transparent and where light will get focused on the retina-> tiny spot (the diameter is in the um range as the incoming light is coherent [nature never expected such light in the first place]) with extreme energy density that can already be in the kW/cm^2 range with a 5mW laser.
UV-A to C (380-100 nm) get blocked by the lens, so they don't get focused and can dispense their energy over a wider area (still NOT recommended) as exposure will lead to cataracts.
So should you use welding goggles or sunglasses that where never designed to block neither coherent light nor light over 400 nm to look at a 405 nm laser? Absolutely effing not.
The Ray Bans exhibit about 3% transmittance at 405 nm, good enough for use with a blue laser pointer IMO unless you plan to aim it directly at your eyes. The Serengetis are OK, too. They have the effect
That's a OD rating of 1.something... Aka: A joke, especially with unknown 100+ mW diodes. You really shouldn't go below 4 or 5.
(Sorry for the rant, but saying that it is safe to look at a laser with anything other than proper laser goggles just doesn't sit well with me)