Monthly:

Big 4 (local): $6400 to $9200 (0-3 PQE); up to $11,500 (5 PQE); $12,500-$13,500 (junior partner) + bonus 1-3 months

International (Magic circle UK): $14,000-$18,000 (0-3 PQE); $20,000-$22,000 (4-5 PQE); $23,000-$24,500 (6-7 PQE)
+ bonus 5-10%

International (non-MC UK): $9,000-$17,000 (0-3 PQE); $13,000-$21,500 (4-5 PQE)
+ bonus 5-10%

International (US): USD $15,000-$17,500 (0-3 PQE if NYC scale); USD $21,600 (5 PQE)
Usually no bonus

Mid tier (local): $4,500-$6,000 (0-3 PQE): $6,500-$8,000 (3-5 PQE); $10,000-$12,000 (above 5 PQE to JP)

Small: anything goes, between $3,500-$5,500 (0-3 PQE)

Source: combination of anecdotal, peers and seniors, & headhunters' salary guides.

How low is low is really up to the individual. Most of my friends won't go below $4,000 unless they're desperate. Depends on your circle of peers.

"Once you start low you remain low" only works in very small firms/solos when you negotiate salary with the sole prop/partners directly. If you join a bigger firm, its based on lock step, with perhaps -1/-2 years discount if you come in from an entirely unrelated practice area.