Rek'Sai Breacher is in my base. If I play a unit that spawns tokens, can I have those enter ready, or do I have to pay a cost to accelerate them?

Yes, the tokens created by your unit will have Accelerate due to Rek'Sai, Breacher's ability, and you must pay the Accelerate cost if you wish for them to enter ready.

Reasoning

1. Rek'Sai, Breacher's Ability: Rek'Sai, Breacher states: "Friendly units played from anywhere other than a player's hand have [Accelerate]." Since tokens are created on the board by an ability (and are not played from your hand), they qualify for this effect.
2. Accelerate Mechanics: According to Rule 731.1.a, Accelerate is functionally short for: "As you play me, you may pay [1] and 1 Power as an additional cost. If you do, I enter ready."
3. Paying the Cost: Accelerate is an optional additional cost. If you want the token to enter ready, you must pay the cost as part of the process of playing the token (Rule 731.2). If you choose not to pay the cost, the token will enter the board exhausted (the default state for units, per Rule 179.1.d).
4. Tokens are Played: Tokens are "played" when they are created by abilities that use that terminology (e.g., "Play a... token"). Per Rule 179.1.a, tokens follow all applicable steps for playing a card, including paying additional costs like Accelerate.

Sequence

• You play a unit that has an ability to create tokens (e.g., "Play a 1[M] Recruit token").
• As the token is being played, Rek'Sai, Breacher's ability grants it Accelerate.
• During the "Pay Costs" step of playing the token (Rule 354), you have the option to pay the [1] and 1 Power cost.
• If you pay the cost, the token enters the board ready.
• If you do not pay the cost, the token enters the board exhausted.

Note: I could not fully verify this ruling against the retrieved rules.