Yes cheapest baby capsule is around £35-40.
The £70 ERF only goes up to 18kg, so wouldn't do if you wanted to rear face up to 25kg. It is suitable from birth though, so you could skip the baby capsule if you didn't need/want the carry factor.
The cheapest 25kg ERF currently is £160. But even RF to 18kg will get you past the most dangerous period, and then there are now seats which harness up to 25kg if your child is too young to booster at 18kg, starting at £80 and mostly converting to booster seats for later.
If you're on a v tight budget you could do the following combos:
Child on low centiles -
Birth - 18kg Joie Tilt £70
Booster seat Britax Adventure £30-40
Total £100-110
Child on average/higher centiles, some ERF:
Birth - 18kg Joie Tilt £70
Harness 25kg + booster Cozy n Safe Hudson £80
Total £150
Maxed out ERF:
Birth - 13kg Joie Juva / Nania Beone £35-40
1y - 25kg RF Britax Two Way Elite £160
Booster seat Britax Adventure £30-40
Total £240
There is also a newish seat called BabyAuto Dupla, which is suitable from birth right to 12 years, ERF to 18kg, Extended harness to 25kg (forward facing) and then booster. Isofix too. I do not know how it performs safety wise, and I don't know how well it fits either newborns or a booster aged child, but it's £140 and I'd definitely use it for the middle stages.