Someone

router bgp 300
 no synchronization
 bgp log-neighbor-changes
 network 100.0.0.0
 network 150.0.0.0 mask 255.0.0.0
 network 200.0.0.0 mask 255.0.0.0
 neighbor 3.1.1.1 remote-as 200
 neighbor 3.1.1.1 route-map out out
 no auto-summary
!
access-list 90 permit 200.0.0.0 0.255.255.255
!
route-map out permit 10
 match ip address 90
 set as-path prepend 300 300