im going to have to look into these cheap modules, i dont usually look into parts/quotes for customers (i work for an IT consulting firm) but we just bought 14 long haul cisco sfp modules for a customer at like $650 each. mind boggling.
Long haul (Single mode) do tend to be more expensive than MM - due to the lower volumes and need for higher precision optical components.
That said, there is a big difference in price range; Cisco will charge hugely more than other vendors for an equivalent part; plus there are variable amounts of vendor lock-in, where a switch will verify the manufacturer and serial number of an optics module, and refuse to initialize it (even if the module is fully standards compliant) if the module is from a different manufacturer.
A friend of mine used to be a reseller. He would frequently get orders for Cisco SFPs. He's order some generic ones from Hong Kong which had their EEPROMs coded to work with cisco gear at $20 each, and sell them for $200. When customers were offered a genuine cisco for $800 each or knock-off for $200, they would usually jump for the knock offs. The knock-offs were also fairly reliable, as the reseller themselves offered a 12 hour swap-out service on their own-brand ones, but as far as my friend knew, they never actually had to swap one out.
I've got a bunch of networking equipment lying around at home, much of it with SFP ports, so just for kicks, I bought a bunch of modules off ebay. Short haul gigabit MM modules can be picked up for pennies. I picked up a 10 pack of MM 4 GB fiberchannel modules (which also support gigabit) for about $10 + $5 shipping, and they've worked great for years in my home LAN. Long haul gigabit ones come up more rarely, but you can get them for $20-50 used.
Just for note, I have some netgear switches including a GS724T v3 (pretty much identical to the OP's switch). It's very happy with Intel 4GB fiber channel MM SFPs.
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