diff --git a/crates/rbuilder/src/building/mod.rs b/crates/rbuilder/src/building/mod.rs
index 84d2c59a7..297aab60e 100644
--- a/crates/rbuilder/src/building/mod.rs
+++ b/crates/rbuilder/src/building/mod.rs
@@ -70,7 +70,7 @@ use std::{
 };
 use thiserror::Error;
 use time::OffsetDateTime;
-use tracing::{error, trace};
+use tracing::{error, trace, warn};
 use tx_sim_cache::TxExecutionCache;
 
 pub mod bid_adjustments;
@@ -864,6 +864,31 @@ impl<Tracer: SimulationTracer, PartialBlockExecutionTracerType: PartialBlockExec
             }
         }
 
+        // flashbots/rbuilder#296: the proposer payout is sent from the builder
+        // coinbase, so the coinbase must hold at least the transferred value
+        // plus the gas the EVM locks up front (gas_limit * basefee, as the
+        // payout tx sets max_priority_fee_per_gas = 0 and max_fee_per_gas =
+        // basefee). If it does not, the payout tx is not sendable (in the
+        // limiting value-less case the floor is BASE_TX_GAS * basefee). Warn so
+        // the condition is visible up front rather than only as a downstream
+        // payout-tx revert. Checked after the refunds above, which also spend
+        // from the coinbase.
+        let builder_balance = fork
+            .state
+            .balance(builder_signer.address)
+            .map_err(CriticalCommitOrderError::Reth)?;
+        let payout_tx_cost =
+            proposer_payout_tx_cost(value, gas_limit, ctx.evm_env.block_env.basefee);
+        if builder_balance < payout_tx_cost {
+            warn!(
+                builder = %builder_signer.address,
+                %builder_balance,
+                %payout_tx_cost,
+                payout_value = %value,
+                "Builder coinbase balance is below the cost of the proposer payout tx; it will not be sendable"
+            );
+        }
+
         let tx = create_payout_tx(
             ctx.chain_spec.as_ref(),
             ctx.evm_env.block_env.basefee,
diff --git a/crates/rbuilder/src/building/payout_tx.rs b/crates/rbuilder/src/building/payout_tx.rs
index 24428c433..8c005e805 100644
--- a/crates/rbuilder/src/building/payout_tx.rs
+++ b/crates/rbuilder/src/building/payout_tx.rs
@@ -39,6 +39,17 @@ pub fn create_payout_tx(
     signer.sign_tx(tx)
 }
 
+/// Minimum builder (coinbase) balance required to send the proposer payout
+/// transaction: the transferred `value` plus the gas the EVM locks up front
+/// (`gas_limit * basefee`, since the payout tx sets `max_fee_per_gas = basefee`
+/// and `max_priority_fee_per_gas = 0`). A builder holding less than this cannot
+/// cover the payout tx and it will not be sendable; in the limiting case of a
+/// value-less payout the floor is `BASE_TX_GAS * basefee`. See
+/// flashbots/rbuilder#296. Saturating so an implausible input cannot panic.
+pub fn proposer_payout_tx_cost(value: U256, gas_limit: u64, basefee: u64) -> U256 {
+    value.saturating_add(U256::from(gas_limit).saturating_mul(U256::from(basefee)))
+}
+
 #[derive(Debug, thiserror::Error)]
 pub enum PayoutTxErr {
     #[error("Reth error: {0}")]
@@ -287,4 +298,53 @@ mod tests {
         assert_matches!(estimate_result, Ok(_));
         assert_eq!(estimate_result.unwrap().gas, 21_000);
     }
+
+    #[test]
+    fn payout_tx_cost_covers_value_and_gas() {
+        let basefee = INITIAL_BASE_FEE;
+
+        // A value-less payout still needs BASE_TX_GAS * basefee: the "21k gas"
+        // floor from flashbots/rbuilder#296.
+        assert_eq!(
+            proposer_payout_tx_cost(U256::ZERO, BASE_TX_GAS, basefee),
+            U256::from(BASE_TX_GAS) * U256::from(basefee),
+        );
+
+        // The transferred value is added on top of the gas cost.
+        let value = U256::from(7_000_000_000u64);
+        assert_eq!(
+            proposer_payout_tx_cost(value, BASE_TX_GAS, basefee),
+            value + U256::from(BASE_TX_GAS) * U256::from(basefee),
+        );
+    }
+
+    #[test]
+    fn payout_tx_cost_matches_guard_boundary() {
+        let basefee = INITIAL_BASE_FEE;
+        let value = U256::from(7_000_000_000u64);
+        // Independently computed cost (NOT via the function under test) so the
+        // assertions below depend on proposer_payout_tx_cost's real output.
+        let exact = value + U256::from(BASE_TX_GAS) * U256::from(basefee);
+
+        // The guard in insert_refunds_and_proposer_payout_tx warns iff
+        // builder_balance < proposer_payout_tx_cost(...). A balance of exactly
+        // the cost is sufficient (no warn); one wei less is insufficient.
+        assert!(
+            exact >= proposer_payout_tx_cost(value, BASE_TX_GAS, basefee),
+            "balance equal to the cost is sufficient",
+        );
+        assert!(
+            exact - U256::from(1) < proposer_payout_tx_cost(value, BASE_TX_GAS, basefee),
+            "balance one wei below the cost is insufficient",
+        );
+    }
+
+    #[test]
+    fn payout_tx_cost_saturates() {
+        // Implausible inputs must saturate rather than panic on overflow.
+        assert_eq!(
+            proposer_payout_tx_cost(U256::MAX, u64::MAX, u64::MAX),
+            U256::MAX,
+        );
+    }
 }