In tabletop it takes roughly 9 guardsmen to kill a marine with one round of shooting at rapid fire range, though that number is theoretically higher since the marine is likely to have gotten a chance to shoot at least once before hitting that range and is far more likely to kill a guardsman with any given shot. In close combat things get slanted even more in the marine's favor.
In terms of points a marine is a little over 3 guardsmen in points cost, which is quite efficient from the marines perspective for what you're getting, but you have less heavy/special weapons in the same slot and less overall wounds.
First rank fire, second rank fire, 12" range, 3 shots per guardsmen hitting 50% of the time. A third of those hits will be severe enough to trigger an armour save (assuming of course, the space marine is not in a bunker and so would not get a 2+ cover save). Two thirds of those saves will be made. Thus at optimal fire discipline with only lasguns and "standard" Imperial Guardsmen, in daylight conditions with no smoke popped, it takes 1.5 pgto hit, 0.5pg to wound, 0.16 recurring to kill. Thus at the best possible condition with an alive officer capable of giving commands (and succeeding), it takes 6 guardsmen to have a probability of 1 to take down 1 standard space marine in optimal conditions.
A standard space marine rapid firing their boltgun gets off 2 shots per space marine, hitting 66% of the time. Boltguns wound guardsmen 66% of the time, and punch through flak armour giving no armour save. In the same optimal firing conditions, a space marine will kill 1.13 guardsmen every round of shooting. In battlefield conditions guardsmen usually almost always are in cover, or at least get cover saves from using other guardsmen units as cover, but the space marines have an obvious advantage in that they never need cover vs lasguns whilst the guardsmen do vs bolters, greatly reducing the mobility of guardsmen vs space marines.
In close combat (assuming the space marines are on the charge, the much more likely scenario as shooty guardsmen firing line is better than charging guardsmen), the guardsman has 50% chance to hit the space marine and 1/3 of a chance to wound them, with armour saves reducing that chance to kill by another 1/3. The space marine on the charge with 2 attacks has 2/3 of a chance to hit the guardsman, 2/3 to wound them, but their flak armour (and helmets!) reduces the odds of death by 1/3.
Thus in the first round of the charge it takes 18 guardsmen to kill 1 space marine with a probability of 1, whilst the space marine is killing 1.68 guardsmen in the charge. Given the obvious discrepancy in casualties this would produce, it would stand a good chance of forcing the guardsman to retreat, and if they fail to escape combat, have the squad entirely destroyed in the space marine's sweeping advance.
So at optimal range and command, 6 guardsmen are worth 1 space marine, and 1 space marine is worth 1.13 guardsmen.
If the space marines charge then 18 guardsmen are worth 1 space marine, and 1 space marine is worth 1.68 guardsmen (not including the possibility of completely wiping out the guardsmen in a sweeping advance).
Those are just regular tactical squad marines without close combat weapons. Space marines like close combat because a lasgun is at the end of the day deadlier than a bayonet
Also I must add, there is an immense joy in rolling 150 dice with a maxed out conscript squad versus space marines. The dice never end.
Basically at the end of the day if the guardsmen want to win, they have to stack the deck in their favour at every opportunity. Autocannon teams guarded from assault squads by the Imperium's finest guardsmen, entrenched in cover flanked by veteran squads equipped with grenades, and leman russ battle cannons firing non-stop, basilisks dropping pie templates like it's hot. If worse comes to worse: Deploy ogryns
*EDIT
That's using the Imperial Guard, which for a "baseline" standard infantry are fluffwise, the best equipped and most disciplined human fighting force that doesn't wear power or carapace armour. A Planetary Defence Force equipped with autoguns or shotguns instead of the Imperial Guard and their lasguns is fucked at range and fucked in close quarters combat vs a space marine