Hello,
The Chris Thrale Webley Air Rifle 'Bible' identifies Webley Mk3 Ser No 28184 as an early Series 4 example introduced in 1957 and available up until 1961. The lowest Series 4 seen Ser No being 28072 and the highest 39352. This would consequently date your earlier Mk3 to 1957.
Dating the A Series rifles is not so easy as for some reason Webley used the A prefix twice with the first batch starting in June 1966 with Ser No A0060 and ending in Nov 1969 with Ser No A9999. The second batch carrying the A prefix started in Nov 1969 beginning at A1 and going up to Ser No A10000 in 1972. If your rifle was part of the second batch then it would date to September 1971.
Accurately estimating values is difficult without some pictures as condition is everything. Ballpark prices for Mk3s would be in the region of £120-£350 and maybe a bit more depending on condition and Production Series.
Hope this helps a bit.
Regards
Brian