Whilst it's more noticeable on the MML, especially since Wellingborough is now only served by the Corby services, similar jumps occur pretty much everywhere upon crossing the Network Area boundary.
GEML: Manningtree - London Terminals: £6208 Annual, £62.30 Anytime Day Return; Ipswich - London Terminals: £7124 Annual, £86.40 Anytime Return
ECML: Huntingdon - London Terminals: £5748 Annual, £49.70 Anytime Day Return; Peterborough - London Terminals: £8436 Annual, £121.20 Anytime Short Return (£7116 Annual, £64.70 Anytime Return for the GN&TL Only option)
WCML: Long Buckby - London Terminals: £6096 Annual, £65.80 Anytime Day Return; Rugby - London Terminals: £8840 Annual, £149.80 Anytime Return
Chiltern: Banbury - London Terminals: £6276 Annual, £86.90 Anytime Return; Leamington Spa - London Terminals (via Banbury): £8724 Annual, £119.00 Anytime Return
GWML: Didcot Parkway - London Terminals: £5544 Annual, £70.30 Anytime Day Return; Swindon - London Terminals: £9512 Annual, £145.60 Anytime Return
Berks & Hants: Bedwyn - London Terminals: £5544 Annual, £66.60 Anytime Day Return; Pewsey - London Terminals: £6944 Annual, £126.60 Anytime Return
Going way back to NFM64 (September 1996) for the examples given back in post 70, plus Rugby:
Bedford Stations - London Terminals: £2708 Annual, £25.10 Standard Day Return
Wellingborough - London Terminals: £3808 Annual, £31.00 Standard Open Return
Northampton - London Terminals: £2836 Annual, £31.60 Standard Day Return
Rugby - London Terminals: £4448 Annual, £42.50 Standard Open Return
Portsmouth Stations - London Terminals: £2932 Annual, £35.10 Standard Day Return