I had to read the English version twice (along with the Irish) to understand it properly, but maybe that was just me .
My first construal of the Irish version was "... to ask the students for a book that costs less than..." rather than "... to require of the students a book that costs less than...". Since "rud a iarraidh ar dhuine" can be "to ask someone for something" or "to ask/require something of someone" according to context *("to do something" if it's a verb, of course), it might be less ambiguous - in both languages - to stick a verb such as "have/get/buy" in there?
Níl cead agam iarraidh ar na scoláirí leabhar atá níos daoire ná $30 a bheith acu (/a fháil/a cheannach).
"I'm not allowed to require/ask the students to have (/get/buy) a book that costs more than $30."
Or am I, as a learner, seeing potential ambiguities which a native speaker would take in his/her stride? What do you think, Bríd?
*(Obviously there's no real difference between "ask someone for help" and "ask help of someone".)