Ertan´ın borçu kaç liraydı? 

First let me say I now have my own copy of Geoffrey Lewis´ Turkish grammar, and already loving it. However i can´t yet find where it explains the structure of affixing -(y)di to a noun (lira). In this context, "Lira" is plural I presume. does that have importance in how it is suffixed?

My guess was that it asks: "How much lira was Ertan´s accomodation?" 

Though if this is incorrect, then I have misunderstood of course. I´m sure the book does explain it somewhere, but I trust the tutors on this site so much because it seems there is still much Turkish that is left up to instinct of which mine still feels weak.

Ertan´ın borçu kaç liraydı?

How many liras was Ertan´s loan? 

When you use ´kaç´ you are asking ´how many´, so just like you don´t use the plural suffix (lar/ler) after a number [eg. beş kişi], you don´t use plural suffixes after ´kaç´. You assumed correctly. 

Kaç gün kalacaksın? = How many days will you stay?

Page 96 explains the forms for the old idi (suffix for the past, was ). Just like ile (with), it could be written separately, but apparently it is no longer used separately since the 1960´s.

The rule is that when added after a vowel, a buffer letter ´y´ replaces the first ´i´ in idi, and vowel harmony (ı,i,ü,u) applies, followed by the appropriate personal ending. The d to t mutation also occurs after specific consonants.

Hasta (sick) + idi (past) + m (1st person) = hastaydım (I was sick)

Geçen yıl İstanbul´daydım (I was in Istanbul last year)

See this lesson

Thanks for the confirmation on -kaç Though I didn´t make the sentence up. Its actually part of a series of questions to answer about someone traveling and staying at a hotel. Is there a better Turkish word for accomodation?

I understand about the use of -di and how you would apply the -y to words ending in a vowel. Your examples use Hasta, which is an adjective**, (someone can physically be ill) and Istanbul, which is a place name noun. ( You can physically be there) Lira is also a noun but its not something a person can be "in a state of" or a "place someone can be". Mr. Lewis´ examples are also adjectives and place nouns. 

Thus my confusion as to why it takes a suffix normally reserved for verb stems. I suppose there is just something I´m missing here.

** unless its referring to a sick person, or patient.

I guess you are confused because they look similar, but there is a difference. The now defunct verb for "to be" was imek. My understanding is that the past tense of ´imek´ (when you remove the infinitive ´mek´ suffix) is ´idi´. This is different to the past tense suffix ´di´. Because ´idi´ is no longer written separately, it is not as obvious to learners. Maybe you could think of this suffix as an attached verb.

Don’t you bother your pretty head with the distribution between nouns and adjectives, Mavili. They are all the same. hasta means as well ‘a sick person’ as ‘sick’. They are both nominal phrases. That’s what is important.

Because.

Because sentences are either verbal or nominal. In a verbal sentence the predicate consists of a verbal phrase:

         Ali koş|tu. = SUBJ + verb + tense marking

In a nominal sentence the predicate consists of a noun phrase plus a copula. The copula is the verb ‘to be’ with no independent meaning of its own, just connecting this nominal phrase to the subject.

         Ali postacı|ydı. = SUBJ + noun + copula

The Turkish copula in nominal sentences is the old *ermek > i –mek which in the past tense takes the form idi. Henry explained the rules which you have to follow when attaching idi to the nominal phrase.

Basically it is a question of sentence type, whether you are telling what the subject is doing (verbal sentence) or what the subject is (nominal sentence). I try to keep these two separate in my mind not to mix with the constituents.