The only thing I know the normal prices of are car seats, so I've gone through all the car seat deals and these are my opinions/picks. Long, waffly version.
Will follow up with summary version of the best options.
RF/FF options up to 105cm/approx age 4:
Graco Extend LX - RF/FF, belt fitted - good for holidays/spares/non drivers. Down to £68 (usually £90)
Graco Turn2Me i-size - £127, usually £160 - v similar model to Joie 360 Spin GTi. This is a really great price for a spin seat with good safety.
Maxi Cosi Pearl 360 - note! only works if you already have the 360 Family base, and personally I would price /feature match with other spin seats, but a good price for this if you were likely to buy it anyway - £195 usually £250-260.
Forward facing car seats with shield or harness:
Cybex Pallas G i-size - shield up to 105cm, HBB to 150cm. £150 (usually £160-170)
Maxi Cosi Titan S Comfort i-Size - Harness to 105cm, HBB to 150cm - good safety rating. £140, usually £180-200.
Maxi Cosi Nomad Plus - portable forward facing folding travel car seat, 76-105cm. £100, Usually £130-140. Note, only FF and may not be suitable for larger toddlers or all vehicles.
Enfasafe Spirit iX i-Size - note, I don't generally recommend this seat, I would spend more if you can, or look at the Graco Extend/Eversure instead, depending on what stage you need. But it is a very cheap price for a FF 123 seat with isofix and top tether.
High back boosters:
Maxi Cosi Rodifix M i-size - £80 down from £100
Graco Eversure i-size - £46, usually £80 (this is a copy of Joie i-Trillo LX)
Britax Discovery Plus 2 - £95, usually £110 (isofix version of Britax Adventure)
Not so good deals, even though amazon highlight them:
Because it's commonly this price:
Graco SnugEssentials infant carrier - also, not compatible with most pushchairs.
Graco Slimfit R129 - a good seat, but often discounted.
Cybex Solution B2 i-Fix - a good seat, but not much of a discount.
Cybex Solution S2 i-Fix - a good seat, but often similar price.
Maxi Cosi Rodific S i-Size - not that much of a discount.
Because I think the seat itself is not great or there are better options for this price point:
Reecle 360 Swivel 0+/1/2/3 seat - cheaply made, hard to use and questionable safety (check the negative reviews).
Jovikids ISOFIX for 0-12 years - same story.
Graco Energi R129 - doesn't offer much compared with others in this category.
Graco FlexiGrow R129 - not great compared with others in this category.
Graco Affix i-size - the Eversure is a better deal.
Cybex Solution X-fix - outdated model, you can get better for less now.
Graco Affix R44 - not much of a discount, and an outdated/poorly performing model.
Pf.Ebro I-Size booster seat - the belt guides are too far forward so it is unlikely to give a good belt fit.
Globalkids i-size High Back Booster - belt guides poorly placed, poor belt fit.
Globalkids Booster seat with isofix - belt guides tricky to use and can get other similar things for this price.
Jovikids i-size booster - similar to other things for the same price.