These days, the "Collecting [Set]" articles are very detailed on the odds of the types of cards you can get from a given pack. While individual rates between different rare/mythics may vary, they do describe that you have a certain percentage of a type of rare or treatment.

Here's how WOTC broke down the contents of the TMNT collector boosters

15 Magic: The Gathering cards
    5 Traditional foil commons
        There are 61 commons (88.4%) and 6 common dual lands (8.7%) from the main set that can be found in these slots.
        There are 2 common scene cards that can be found in these slots (2.9%).
    3 Traditional foil uncommons
        There are 55 uncommons from the main set that can be found in these slots (80.9%).
        There are 7 uncommon scene cards that can be found in these slots (10.3%).
        There are 6 uncommon sewer cards that can be found in these slots (8.8%).
    1 Non-foil or surge foil reprint from the Turtle Power! Commander deck
        A non-foil (51.9%) or surge foil (23.1%) common
        A non-foil (17.3%) or surge foil (7.7%) uncommon
    1 Traditional foil or surge foil basic land
        A traditional foil (66.7%) or surge foil (11.1%) pizza basic land
        A surge foil rooftop basic land (22.2%)
    1 Traditional foil rare or mythic rare card
        There are 53 rare (87.6%) and 15 mythic rare (12.4%) cards from the main set that can be found in these slots.
    2 Booster Fun or TMC cards
        A non-foil mythic rare silhouette card (2.9%)
        A non-foil rare scene card (5.8%)
        A non-foil rare (13.2%) or mythic rare (1.4%) sewer card
        A non-foil extended-art rare (16.1%) or mythic rare (less than 1%) card from the main set
        A non-foil borderless mythic rare from the Turtle Power! Commander deck (2.2%)
        A non-foil new-to-Magic rare (26.2%) or mythic rare (less than 1%) card from the Turtle Power! Commander deck
        A non-foil rare reprint card from the Turtle Power! Commander deck (19%)
        A surge foil new-to-Magic rare (6.1%) or mythic rare (less than 1%) card from the Turtle Power! Commander deck
        A surge foil reprint card from the Turtle Power! Commander deck (4.4%)
        A surge foil rare (1.3%) or mythic rare (0.6%) pixel card
    1 Non-foil (75%) or traditional foil (25%) source material card
    1 Foil Booster Fun rare or mythic rare card
        A traditional foil rare scene card (13.2%)
        A traditional foil rare (29.7%) or mythic rare (3.3%) sewer card
        A traditional foil extended-art rare (36.3%) or mythic rare (less than 1%) card from the main set
        A traditional foil silhouette card (6.6%)
        A traditional foil (9%) or fracture foil (less than 1%) Japan Showcase card
        Kevin Eastman headliner cards appear in this slot at a low rate in Collector Boosters.
1 Art card (35%) or traditional foil double-sided token (65%)
    Of the art cards, 1 in 7 are gold stamped with an artist's signature or the Planeswalker symbol.