Maker-taker economics for grid bots: when post-only actually pays

Grid bots don't win by predicting direction — they win on fee bps. Every rung you fill is one leg of a round trip, and a grid that trades 400 times a day turns a 1 bps fee gap into real money. The question isn't "maker or taker" in the abstract; it's whether forcing post-only actually earns its keep once you count the fills you miss by insisting on being a maker.

Here's the trade-off in one line: post-only guarantees the maker rate but rejects any order that would cross the book, so in fast markets some rungs never fill and you lose that grid step. A taker order fills every time but pays the higher rate.

The break-even is easy to model. Let:

• m = maker fee (bps)
• t = taker fee (bps)
• p = probability a post-only rung actually fills before price walks away
• g = grid step captured per successful round trip (bps)

A post-only rung's expected value per attempt is p * (g - 2m). A taker rung's is g - 2t (it always fills). Post-only wins when:

def post_only_wins(g, m, t, p):
    ev_maker = p * (g - 2 * m)   # fills with prob p, pays maker both legs
    ev_taker = (g - 2 * t)       # always fills, pays taker both legs
    return ev_maker > ev_taker

# 5 bps grid step, maker 2 bps, taker 5 bps
for p in (0.6, 0.72, 0.9, 1.0):
    print(p, post_only_wins(5, 2, 5, p))
# 0.60 False
# 0.72 True (right at the edge)
# 0.90 True
# 1.00 True

With a 5 bps step and a 2/5 maker-taker split, post-only stops paying off once your fill probability drops below ~0.72. On a tight, liquid pair where post-only fills 90%+ of the time, maker-only is clearly correct. On a thin pair during a move, where half your post-only rungs get rejected, you're better off just taking.

Net cost per round trip (both legs) at a few fee tiers:

maker/taker (bps)	maker round trip	taker round trip	crossover fill-prob (g=5)
2 / 5	4 bps	10 bps	0.72
1 / 4	2 bps	8 bps	0.60
0 / 5	0 bps	10 bps	0.50

The practical takeaway: measure your actual post-only fill rate per pair — most bot frameworks log rejected orders, so count them — plug it in, and stop assuming maker-only is free. It isn't; the hidden cost is the fills you never got.

I keep the raw per-exchange fee tiers I use for this in a public dataset: https://github.com/jack0752168/crypto-exchange-fee-data

A longer write-up on grid fee optimization with worked examples is here: https://www.jacktrader.xyz/en/blog/grid-bot-fee-optimization.html

Not financial advice — just the arithmetic. Your fill rates and fee tiers are yours to measure.