Rob Jansen, Tavish Vidya, and Micah Sherr have a paper about bandwidth-based DoS against Tor. Section 5 is about default bridges.
https://www.usenix.org/conference/usenixsecurity19/presentation/jansen https://www.usenix.org/system/files/sec19-jansen.pdf#page=6
While elsewhere in the paper they discuss in-protocol attacks, in the context of bridges they limit themselves to attacks using third-party paid "stresser" DoS services, which you can rent for about $1/Gbps/hour (Section 3.1).
They looked at the default bridges in Tor Browser 8.0.3 (October 2018). Only 12 of 25 default obfs4 bridges were working. (I think most of the non-working bridges have since been pruned, e.g. in #29378, #30264.) The median bandwidth of the 12 working bridges was 368 KB/s, with a large variance: minimum of 67 KB/s and maximum of 1190 KB/s.
Besides the default bridges, they requested 135 bridges from BridgeDB, and found that only 70% (95/135) of them worked. (This has also been at least partially addressed by #30441.) BridgeDB bridges are faster than default bridges, with a median bandwidth of closer to 600 KB/s (Figure 1).
They estimate that disabling the 12 default obfs4 bridges would require 30 stresser jobs, at a rate of $22/hour or $17K/month. If all users of default obfs4 bridges switched to BridgeDB bridges, the median bandwidth of BridgeDB bridges would slow down to under 100 KB/s (Figure 2). If even half of default obfs4 users switch to using meek, the cost to operate meek will at least double (Figure 3).