The introductory notes to the Society of Indexers' Indexers available will be helpful here. If you have a specific size in mind, tell the indexer how many lines you have - indexers would prefer to edit an index down themselves. Do not expect a cheaper index because you have insufficient pages to spare for it. Indexing involves making as many entries as needed in an ideal situation and then editing - it cannot be done on an ad hoc basis. If the lines are fairly long, there may be so many turnovers in a thin column width as to create confusion for the reader.
Every text is different.
If you reckon an approximate figure of 8-10 pages per hour for straightforward text, you will arrive at an estimate for the length of the job. Once the indexer has seen the proofs, this estimate may have to be revised. The indexer should return the index to you within the specified period. If you expect an index to be done particularly quickly, involving evening or weekend hours, expect a higher fee for the job. The Society of Indexers recommends a minimum hourly rate for indexing.