But then you're mostly guessing and presuming that it's not economical to own your own printers. I don't know about your situation but from a business point of view you need to look much more closely at the options and accurately work out the differences. I say differences because a lot of people only consider price when there are other aspects like P&P, delivery times, print issues, re-prints etc. to take into account.
I think the biggest issue with the labs is the postage cost. If you were to order five A4 prints from DS colour labs you would pay £3.25 for the prints and £3.99 for the postage. If you count for the P&P in the print it works out at £1.44 per print rather than the listed £0.65. I don't know how many images people order in one batch but I deal on a per client basis who as a minimum will order 5-10 prints.
Then there is the issue with the time-scales. The delivery time is reportedly 1-10 days which means you can't provide your clients with an accurate ETA. If your client receives their order and makes another one afterwards which is quite common, you will have to quite up to 14 days including sending them out yourself which is inconvenient. If you're just printing for personal reasons then you will have to wait potentially 10 days. It may be debated whether it's important to receive an order so quickly but in general people respond better to a faster more accurate service.
I think if the labs sort out their postage it may work out a bit better. I know it doesn't cost £4 to send those prints. It costs me about 20p for the envelope and first class large letter is about 94p? They would actually get a discount anyway for franking the mail.
with you mentioning DSCL their estimated times are as follows according to my last order email
Standard prints: Same day if ordered before 1pm Mon-Fri only.
Large format prints and canvas: 1-3 days.
All other products will be dispatched with in a 3-7 day period,
i'm not guessing i have owned a printer of one type or another for the last 29 years purely on a domestic level i fully understand in business speed could be more important than cost and regular orders could work out cheaper to print youself
at this moment in time i own 3 printers an oki C301dn A4 colour laser, an oki C822 A3 colour laser and a rather ageing canon s9000 A3+ inkjet which iv'e owned since launch around 12 years ago
over the years online photo printing has become cheaper and cheaper to a point where it's just not worth doing them yourself, this has been the case for quite a few years and one of the reasons iv'e never bothered updating to a newer inkjet printer
i use laser printers for everything else
never owned a sub dye printer yet but it's on my wish list if ever the opportunity / price becomes a reality
since we are talking about photo prints i will ignore the lasers but lets consider the cost of my last order from DSCL
3 x Prints (10" x 8", Lustre, (Pro Only), No border) @ £1.59
4 x Prints (A3 16.54"x 11.69" (Require Trimming), Lustre, (Pro Only), No border) @ £4.60
1 x Print (A4 11.7" x 8.3", Lustre, (Pro Only), No border) @ 65p
Postage & Packing: £1.80 i live close enough to DSCL to collect but still get charged £1.80 for the privilege
a grand total of £8.64
if i were to print this little lot myself providing i get them spot on first time on canon paper ( which i use ) without having to reprint any, the cost of paper alone would come close to the cost of DSCL printing them
add the price of ink per sheet of paper ( canon originals k) it's cheaper for DSCL to print them
now add the cost of the printer itself
in my personal circumstance it would take a long time if ever to break even not to mention the results are usually better than my ageing printer can produce anyway
if i were to start from scratch and buy a new printer probably one of the current A3+ that canon offer i could get a lot of prints done for the cost of said printer never mind feeding it with decent paper and ink and that's where i'm coming from
the OP's original questions was about occasional printing for domestic use and 30 - 40 A3 prints twice a year and in those circumstances i still think having them printed is cheaper than buying a printer and printing them at home