Buying the Materia is only for the impatient players. You have 75 days time if you login and do your Expert roulette each day. Might be shorter if you are willing to invest your Myth tomes but I guess for the majority it will take longer than 75 days. You do not need to insert them immediately. The Alexandrites stack (Thanks God) and you can even wait till you have them all before you insert the first one.
What I do each day is to equip myself with 3-4 items where I know a Materia I, II or III will come out of it. Then I do the daily beastmen quests. The items with Materia I are bound to 70-80% after doing six quests. Add 1-2 FATES without a moss mob to it, and you have 3-4 Materias in under an hour. Of course, the Materia II or III will take longer but not that long. I craft these items in HQ by myself and insert the cheapest Resistance Materia to speed up the process.
Additonally, I harvest or buy crap items from the market and let my crafters do a mass synthesis while wearing SB items. Even for Materia III items (Level 45+), it takes only 2 x 99 item mass synthesis to reach 100%. This can be done while you surf or do other things. Shards to craft them can be gathered from your retainers so no extra work needed. This way I am getting at least 20 Materia per week. 75/7 = almost 11 weeks time = over 200 Materia. Surely there will be a lot of Earth resistance or sec. stats I do not need and can sell or make other use of them but I am confident that I will get the vast majority of the Materia I need without buying it on the Market board.
Materia IV will be the only ones where you may have to invest money because chance to bind one (and to get the correct one) is quite low. But you can avoid Materia IV to a certain extent if you add more than 2 secondary values. Relly do not see a problem here since you can get almost all the Materia you need by doing things you would do anyways.