There's a video on YouTube where somebody is flying Mobius Mini (~27 grams, see tech specs) strapped to the Tello and seemingly buzzes around unaffected. I think it's probably pushing the Tello a bit though in the long run. I lifted more than that using a custom wire frame that I built that snaps into the legs and keeps things dead center w/o affecting the downward sensors.
I myself opted for the 14g Hawkey Firefly Micro Action Cam (waiting for it to be delivered from China). At $18 (CN) to $24 (US), it is also 'cost appropriate' (if the drone is lost, you'd only be out around ~US$100).
Not sure how to mount the Firefly just yet. The bracket adds too much weight, IMHO. Maybe not only take it out of the supplied angle bracket, but also out of its housing and custom 3D-print a housing acting also as bottom plate for the Tello with the Firefly internals in it, with some sort of adjustable angle mount. It could be a fairly open design to allow for best ventilation of camera and Tello. Luckily, the Tello's downward facing sensors (4) are all in the back and seem to have 'tunnel' vision judging from the recessed mounting of said sensors. That should allow for some good weight balancing, as you don't want to throw the drone off too much, if at all.
Reviews are somewhat mixed regarding the 1080 quality of this tiny Firefly camera. Some prefer the 720 footage it produces, --even after upsampling it to 1080. I think it'll be fine after some light post-processing in DaVinci Resolve and similar. To be determined...
Lastly, another ultra-light option is a key fob camera that created a lot of interest on various RC forums. It's the ORIGINAL #16 HD Keycam . Have fun, it's only 1330 pages of forum discussion. Unbelievable.
It's a 720p camera, but somehow seems to do quite well, as far as I can tell. Once taken out of the key fob enclosure, its weight is probably under 10g.
As always, beware of fakes. I tried to provide URLs to the manufacturers where possible, but it's no guarantee.
Oh and, anyone with a 3D design of the bottom cover? I'd be really interested in that...