Undecim milibus

Livy (adapted by Orberg (cap XLIV)) is describing how the citizens’ (of the early Rome) obligations are decided after a census: ‘…Undecim milibus haec classis censebatur.’ Is this an ablative milibus? How does it work? 'They were estimated at (ablative?) 11,000?

It’s an ablative or price or value, here, the amount at which they were assessed. Sestertiorum is understood.

See Lewis and Short censeo:

A In reference to the census (v. census).
1 Of the censor (v. censor).

(a) Rarely act. with acc. of the persons or objects assessed or rated; but usu. pass., with subj. -nom.: censores populi aevitates, suboles, familias, pecuniasque censento, Cic. Leg. 3, 3, 7: census quom sum, juratori recte rationem dedi, Plaut. Trin. 4, 2, 30: censor ad quojus censionem, id est arbitrium, populus censeretur, Varr. L. L. 5, § 81 Müll.: census… indicat eum qui sit census se jam tum gessisse pro cive, Cic. Arch. 5, 11: absentīs censere jubere, P. Scipio ap. Gell. 5, 19, 16: ne absens censeare. Cic. Att. 1, 18, 8: sub lustrum censeri, id. ib.: milia octoginta eo lustro civium censa dicuntur, Liv. 1, 44, 2: censa civium capita centum septendecim milia trecenta undeviginti, id. 3, 24, 10; id. Epit. lib. 11; 13; 14: censebantur ejus aetatis lustrīs ducena quinquagena milia capitum, id. 9, 19, 2: cum capitum liberorum censa essent CLII. milia, Plin. 33, 1, 5, § 16: quid se vivere, quid in parte civium censeri, si… id obtinere universi non possint? Liv 7, 18, 5.—

(b) > With the amount at which the property was rated, in the > acc.: or > abl> .: praesertim census equestrem Summam nummorum, being assessed with the estate necessary to a Roman knight, Hor. A. P. 383: primae classis homines quicentum et viginti quinque milia aeris ampliusve censi erant… Ceterarumque omnium classium qui minore summa aeris censebantur, Gell. 7 (6), 13, 1 sq.—Hence, capite censi, those who were assessed ac cording to their ability to labor: qui nullo aut perquam parvo aere censebantur capite censi vocabantur. Extremus autem census capite censorum aeris fuit trecentis septuaginta quinque, Jul. Paul. ap. Gell. 16, 10, 10; Sall. J. 86, 2; Gell. 16, 10, 11; 16, 10, 14; Val. Max. 2, 3, 1; 7, 6, 1; and in the finite verb: omnia illius (i. e. sapientis) esse dicimus, cum… capite censebitur, Sen. Ben. 7, 8, 1. —

Many thanks for such a thorough answer.

You’re welcome, but my answer didn’t amount to very much–I just quoted a big chunk of L&S.