This thread has a lot of information on what devices work to repeat Xiaomi devices. See the first post for a summary of what you need to know and devices that are known to work: Xiaomi devices - are they pairing / staying connected for you?
Personally, I have two XBee modules (not only are they great repeaters, you can also map your ZigBee network with them--which people who've figured these things out for Xiaomi devices need to do so they know what works) and one IKEA Trådfri outlet. The Sylvania 72922 that is known to work well is likely no longer available new--anywhere you find that claims to sell one likely has the 72922-A revision, which has been reported to not work (not just that it's unknown how it performs).
If you want something cheap, the IKEA Trådfri outlet is probably where it's at--plus you'll get a usable ZigBee on/off plug out of it (though they aren't reported to have as strong of signal as some others--not surprising given their cheap price and somewhat small size, though they are much bigger than some other outlets). The XBees are great but more work to set up, and all you'll get out of it is a (fantastic) repeater. If you live in the US, IKEA will do "small item" shipping for $9 (can probably fit a few outlets and other small items in the same shipment for that price), which, yes, is about the cost of the outlet itself--but $18 is still less than pretty much any Z-Wave and most ZigBee outlets you'll find. 
As a tip: if you have repeaters that are known not to play well with Xiaomi devices, unplug them during pairing. In my experience and others', Xiaomi devices are reluctant to change routes ones they get one (it will likely be the hub if you follow the old advice to pair it right next to the hub, but I've paired mine close to XBees or good repeaters instead with great success). So once your Xiaomi device gets a route that works for it (either the hub or a good repeater), plugging in another repeater is unlikely to affect it.
